Make sure the playing notification is only shown when playing is
actually started.

Change Icba3939b87df72986f56e2305cbc59df9feb20f9 introduced a regression
in notification behaviour: As the album art is fetched from onCreate()
(via onCreate() -> reloadQueue() -> openCurrentAndNext() ->
updateAlbumBitmap() ) and the album art fetching code unconditionally
posts the notification, the notification was also posted under a number
of unwanted conditions:

- Starting the app, but not starting playing
- Disconnecting A2DP or headset with nothing playing (service was
  started via the ACTION_AUDIO_BECOMING_NOISY intent in that case)

Fix that and also make sure pending image fetches are cancelled when the
service is shut down.

Change-Id: Id1bff34370e64faf3557bc4587bbb81d47ca9afc