Make AndroidGamePlatform.Exit() to finish current activity.

This is default back key press behavior (http://developer.android.com/reference/android/app/Activity.html#onBackPressed%28%29). Previously activity was kept alive.
This PR partially reverts https://github.com/mono/MonoGame/pull/2813

My game has the exit yes/no dialog. When user confirms exit and relaunches the game, exit dialog is still visible.