android: Ask user to add our app to the device's power whitelist

This is necessary so we can actually schedule events accurately in Doze
mode. Otherwise, we'd only get woken in intervals of several minutes (up to
15 according to the docs) after about an hour.

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

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <!--
-    Copyright (C) 2012-2018 Tobias Brunner
+    Copyright (C) 2012-2020 Tobias Brunner
     Copyright (C) 2012 Giuliano Grassi
     Copyright (C) 2012 Ralf Sager
     HSR Hochschule fuer Technik Rapperswil
@@ -22,6 +22,7 @@
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
     <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
+    <uses-permission android:name="android.permission.REQUEST_IGNORE_BATTERY_OPTIMIZATIONS" />
 
     <application
         android:name=".logic.StrongSwanApplication"