Android: OnDestroy isn't called when I close the app from the recent apps button

As specified in the Android documentation, it is not guaranteed that onDestroy() will be called when exiting your application.

"There are situations where the system will simply kill the activity's hosting process without calling this method"

https://developer.android.com/reference/android/app/Activity.html#onDestroy%28%29

Instead, you can create a service which will be notified when the Task your activities are running inside is destroyed.

Create the service class:

public class ClosingService extends Service {

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public void onTaskRemoved(Intent rootIntent) {
        super.onTaskRemoved(rootIntent);

        // Handle application closing
        fireClosingNotification();

        // Destroy the service
        stopSelf();
    }
}

Declare / register your service in the manifest (within the application tag, but outside any activity tags):

<service android:name=".services.ClosingService"
             android:stopWithTask="false"/>

Specifying stopWithTask="false" will cause the onTaskRemoved() method to be triggered in your service when the task is removed from the Process.

Here you can run your closing application logic, before calling stopSelf() to destroy the Service.