[WP8] Fix for music playing after tombstoning and restoring the app, whe...

...n already stopped.

To recreate without this fix:
If you start to play a song via MediaPlayer, stop it, then tombstone you app. Once you go back to your app and it restores, the song you previously stopped will begin to play again without any calls to .Play(). Trying to stop it with .Stop() did not work at all.

This applies to Windows Phone 8 only.