Fix android game platform exit

This reverts #2789 since it wasn't the proper fix for Android since it was moving the task to the background in all cases without letting the user code to handle its own logic for the back button.

After further investigation is seems that the only viable option (that doesn't involve new public API to handle this case) is to change the Game.Exit behavior on android to always call Activity.MoveTaskToBack() since actually exiting the app is usually frowned upon. The app code should decide what to do when Gamepad.Back is set to true (call Game.Exit or navigate the game menu stack).

For some reason AndroidGameActivity.cs below is not cleanly merged; I tried to redo this pull request a couple times with the same result; this is due to another PR that was merged after #2789. The only thing that changed in there is OnBackPressed() handler being removed.