Android: Add option to change the background running property during runtime

If an application needs to continue to handle signals when it is in background then the Android manifest has to be altered which adds the
QT_BLOCK_EVENT_LOOPS_WHEN_SUSPENDED=1 environmental variable:

<meta-data android:name="android.app.background_running" android:value="true"/>

Under some circumstances it is very useful to only temporary enable the background signal processing. For instance when music is playing then the app might need to continue to receive signals.