Don't unmute when trying to show volume UI. DO NOT MERGE

When a MediaSession is active the adjust volume event to show UI was
causing the stream to be unmuted. Since this happens after every mute
event you were unable to mute while a session was active. This change
doesn't unmute for events that just show the UI.

bug:18844550
Change-Id: Ic8b0f1ab1354646724ead4572a973c302c275eab