fix volume handling

without this patch, when the screen is on and the "Volume button music control"
enabled, subsequent single volume key presses are handled twice - the volume
goes up and down in bigger steps then it should

- handleVolumeKey should be called only when the action is not passed
to the music activity

Change-Id: I974a0cce7b1deae43f2f3d37b8e492dfca250b76