Allows the user to switch between performance and battery saving modes (#422)

* add game_mode_config in mainfest

* add ``game_mode_config``

src/pandroid/app/src/main/AndroidManifest.xml

@@ -20,6 +20,10 @@
         android:hardwareAccelerated="true"
         android:theme="@style/Theme.Pandroid"
         tools:targetApi="31">
+
+        <meta-data android:name="android.game_mode_config"
+            android:resource="@xml/game_mode_config" />
+        
         <activity
             android:name=".app.MainActivity"
             android:exported="true"

src/pandroid/app/src/main/res/xml/game_mode_config.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<game-mode-config
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:supportsBatteryGameMode="true"
+    android:supportsPerformanceGameMode="true"
+    android:allowGameDownscaling="false"
+    android:allowGameFpsOverride="false"/>