[WP8] Fix missing deactivated event when closing the game

Up until now the Deactivated event was not fired on Windows Phone 8 when the game is closed using Game.Exit() (e.g. back button was pressed). This PR fixes this issue.

http://msdn.microsoft.com/en-us/library/windowsphone/develop/system.windows.application.terminate%28v=vs.105%29.aspx

> The Terminate method does not raise any application lifecycle events. Before you call this method, you must save the state of the app.