service: Avoid null intent onStartCommand().

Null intent will be sent to services in the case of some crashes.
MPD.java: Fix Add & Play.

This fixes a small bug added (abd63e8) where Add & Play broke.
NowPlaying*Fragment: If stopped and the playlist changes, initiate a update.

If stopped, and the playlist changes, initiate a track & cover information
update.
MPDroidService: Before stopSelf(), stopForeground() needs to run.
dmix: Update to SDK 20.