[WP8] Fixed missing exit call.

This is a fix for a missing event trigger during exit on WP8.
This also seems to fix the crash/exception that happens right after
exiting the game due to the audio engine not being shutdown/disposed
correctly, since now it does.

Game.Exiting event also works now if Game.Exit() is used. This wasn't
the case before.

Since the raised event appears (so far) to run on the same thread as the
Application.Terminate() call, it seems all gets as much time as needed
to run all of the exit code before actually terminating.

This wasn't the case previously for anyone who tried to handle exiting
of their game manually (due to Game.Exiting not working on WP8
previously).