game_engine. Add ad event handling.

Add an option for the game to enable ads using a key in the game bunle.
Add ad listener to respond to ad events.
Only show the ad after it has been loaded.
Hide the add if it is not loaded.
game_engine. Cache game bundle.

Change initialize ad and initialize game view to use cached bundle.
Check ad enabled flag in the bundle outside initialize ad.
This is a preparation step toward adding the option to enable and disable the ad during the game.
game_engine. Add enable/disable ad to game activity.
g_race. Enable ads in g-race.

This change is required, since the game now expects the ad enabled flag in the game bundle to enable ads.