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Use .getWindow() in Application-file

This may be a silly question but I have the following situation: I want to setup my window layout every time the app is open, for example changing the status bar color. So I created my Application file because I heard it is better if you check something like this in the application file and not in the MainActivity.
Problem: How can I call the getWindow() method without an open activity.
Thanks for helping.

A bit tricky, but you can use Window manager in background-service to display your views without opening an Activity.
Also, for API Level 26+ (Oreo or above) you have to start your service as foreground service

How to close old activity before opening next?

I am building an Android app and are using the Parse SDK for Android.
Currently I am subscribing to channels with this in my main activity:
PushService.subscribe(this, "User_1_channel", MainActivity.class")
This works fine except for one thing. It opens up MainActivity and it gets put upon the other MainActivity in the stack which is very bad. I really want to remove the old activity before the new launches. I know I can do it with this but I have no idea where to put it since the Parse SDK handles everything.
launch.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
People say an alternative is to use a Custom broadcastreceiver. Problem with that approach is I have no idea how to subscribe to a channel without specifying an activity as the third parameter.

If you define the activity as SingleTask in your configuration file, it will indicate only that one activity will be used the Task.
android:launchMode="singleTask"

Launch an application in a different "context"

First of all, sorry for the "context" in the question title, I didn't know which word to use. I successfully launch my app by clicking over a URL from another application, but when I launch the task manager I realize that my application is not actually loaded: the caller app holds the activities. I would like how to:
Launch my app in a different "context" (sorry for the word again, which would be better?)
Be able to reload my app in the case it was already loaded (something like restarting it).
Thank you so much.

You can modify the behavior by setting "launchMode" attribute in AndroidManifest.xml to either "singleTask" or "singleInstance", both would cause your Activity to be created as the root of a new task. However it doesn't restart the Activity if it exist already, instead you should handle the Activity.onNewIntent(Intent intent) callback.
To learn more on launchMode see here: http://developer.android.com/guide/topics/manifest/activity-element.html#lmode

In addition to launchMode that Kai mentioned, you might also want to look at taskAffinity and allowTaskReparenting, depending on how your app is structured.
This is an excerpt from there that seems to match how you describe your app:
For example, if an e-mail message contains a link to a web page, clicking the link brings up an activity that can display the page. That activity is defined by the browser application, but is launched as part of the e-mail task. If it's reparented to the browser task, it will be shown when the browser next comes to the front, and will be absent when the e-mail task again comes forward.

Creating a login type of activity and forcing it the only opening point

I have been doing my research, but I feel as if I am missing something.
I have an app with a login. Each time you open the app, you should be forced through that login page. You should never be able to resume onto any activity other than the login.
In the manifest I have
android:clearTaskOnLaunch="true"
on the main activity I wish to use as the login activity,
and
android:finishOnTaskLaunch="true"
as well as
android:excludeFromRecents="true"
on the rest of the activities.
The problamatic situation occurs when you go from the login to another activity, hit home, and relaunch the app via the icon. It should jump back to the login page, but it doesnt. Any idea?
I have also been installing as a regular apk, not via eclipse as I know that there is an issue with eclipse and some of the manifest attributes.
Perhaps if there is a way to detect that the activity launch came from the app icon press, I could manage it that way, but I dont think that is possible either.

In either onResume or onRestart you could check a series of flags, such as a login timeout, then force the user back to the login activity using an Intent, while at the same time finishing the original activity.
I like this method in favor or just finishing the app in either onPause or onStop because it gives you a chance to make some checks before blindly closing the application.
Or, you could try using the android:noHistory tag in your manifest file.
A value of "true" means that the activity will not leave a historical trace. It will not remain in the activity stack for the task, so the user will not be able to return to it.
There are also other tags such as, android:finishOnTaskLaunch
Whether or not an existing instance of the activity should be shut down (finished) whenever the user again launches its task (chooses the task on the home screen) — "true" if it should be shut down, and "false" if not. The default value is "false".
More information here: http://developer.android.com/guide/topics/manifest/activity-element.html

The easiest and fastest way is probably to terminate the activity with finish(); in its onPause() since this is invoked when the App is put into the background.
It might be possible to solve with XML configuration as well, but not that I know of by heart.

android - How to check that the Process is alive or not?

I want to check whether the process is still alive or not through programmatically ,Can I do that I am trying to do it by process name in onCreate method but the issue is that the onCreate method is called always .When I check that in onCreate method I always get the process name and I can not kill the current app and switch to previous one.
Can anybody help me ?
Thanks in advance.

I'm not sure I understand why you need this. It sounds more like you are wanting to save your state between application changes (your app going to the background).
On the Android, there are no two programs running at the same time, basically (there are services, but those are different). Once your user goes back to the main screen, if they "launch" your application again, it will go to the first activity defined, unless you override some functions to restore the previous state of the application.

There is no "the previous one" to go back to.
Unless you have seriously messed around with your manifest, there will be only one copy of your application (in one process) at a time.

Refer to this article in the Android documentation, specifically the Processes and Threads section:
Application Fundamentals
When the first of an application's components needs to be run, Android starts a Linux process for it with a single thread of execution. By default, all components of the application run in that process and thread.