Split the Android platform java logic into an Android library module (lib) and an application module (app).

The application module `app` serves double duties of providing the prebuilt Godot binaries ('android_debug.apk', 'android_release.apk') and the Godot custom build template ('android_source.zip').

platform/android/java/app/src/com/godot/game/GodotApp.java

@@ -0,0 +1,10 @@
+package com.godot.game;
+
+import org.godotengine.godot.Godot;
+
+/**
+ * Template activity for Godot Android custom builds.
+ * Feel free to extend and modify this class for your custom logic.
+ */
+public class GodotApp extends Godot {
+}