Fix issue 1883666: Audio coming from the music player stopped suddenly

The problem comes from the code handling the automatic change of audio routing to speaker when notifications are played. The music is also muted while the sound is forced to speaker.
To avoid truncating the end of the notification, a delay is inserted between the end of the notification and the restoration of the audio routing.  If a new notification starts during this delay, the current music mute state read and saved before muting music corresponds to the forced mute due to previous notification. When the new notification ends, the mute state restored is muted and music stream stays muted for ever.

The fix consists in reading and saving music mute state only if the audio routing has been restored (check that mForcedRoute is back to 0).