android: Request permission to display notifications on Android 13

Note that displaying the notification for the background service is
apparently not strictly necessary.  So it's fine if the user wants to
hide it.  That the service is running can still be seen in the task
manager (pull down the status drawer twice, there is a bullet with a number
at the bottom if the service is running).

Simply use the system dialog.  If the user denies it twice, it won't show
up again.  The explanation dialog would not show up the first time (i.e.
shouldShowRequestPermissionRationale() returns false), only once the user
denied the permission once.  Currently seems like a bit much work
as we don't need the user to allow notifications.

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -22,6 +22,7 @@
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
     <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
     <uses-permission android:name="android.permission.REQUEST_IGNORE_BATTERY_OPTIMIZATIONS" />
     <!-- necessary to allow users to select ex-/included apps and EAP-TNC -->