DONE:
- fixed create syntax for RRDB
- fixed loading of previously-saved play position
- prevent RRDB from getting updated if its AsyncTask is already running
- register and unregister MediaStoreObserver; triggers if you add or
  remove music from your device

TODO:
- populate RRDB not just on a change to the MediaStore
- play music based on contents of RRDB instead of MediaStore
- fix selecting next song when current song is finished
- prevent duplicate entries in RRDB