MPDroidService: Do not stopSelf() on persistent service intent.

Do not stopSelf() in onStartCommand() when a persistent service intent is
received. This will lead to negating the persistent service intent and
the service will die after the last client unbinds.
StreamHandler: On audio focus change, the MediaPlayer object might be null.

Due to changes in how we handle the StreamHandler and audio focus, the
MediaPlayer might be null on audio focus gain. In this case, if the
StreamHandler is still active, it will be restarted by MPDroidService.
MPDApplication: Set service persistence on service handler status return.

Set the service persistence on service handler status return, rather than
on service handler execution. This fixes inaccurate service handler status
after onTaskRemoved() execution with an active service handler.