Merged Kevin's changes w/ my changes that got lost.
 - Replaced RoviMediaPlayer wrapper w/ Android's MediaPlayer. No need since all it is is an incomplete pass-through.
 - No longer playback assets. Now it uses the songs on the external storage drive.
 - Added button to rescan the external storage for new music files
 - Added track metadata class
 - Replaced getRandomSongFileDescriptor w/ getRandomSong, which uses the new track metadata class
 - Added tests for MediaPlayerPresenter around finding no audio tracks
 - Added class LocalSourceManager for getting a list of songs from the Android audio mediastore
 - Added tests for LocalSourceManager
 - When a song finishes, the next will (usually) play. Sometimes the onCompletionListener doesn't fire though.