Ensure a playing song is stopped when switching songs.

With multiple songs queued up in MediaPlayer and using the MediaPlayer.Default implementation, my existing Song wasn't being told to stop playing before the next one could start, resulting in a resource conflict or overlapping songs. It looks like in many cases the underlying platforms are enforcing a one-song-at-a-time setup. For those who don't, this should prevent conflicts.

@RayBatts Does this look sane?