game_engine. Change game engine event handler to activity.

The game engine activity interface lets the engine call the activity that runs the engine.
Add the enable/disable ad method to the game engine activity interface.
game_engine. Add access to the activity asset to engine acticity.
game_engine. Pass the java game engine activity to the native engine.

The game engine activity is passed to the native engine platform.
The activty is held as a weak reference by the platform.