avrcp: implement ChangePlayback() method
avrcp: implement ChangePlayback() method

ChangePlayback() is used by applications to inform bluetoothd of the
current status of playback.

pstglia/external-bluetooth-bluez,mapfau/bluez,pkarasev3/bluez,ComputeCycles/bluez,pkarasev3/bluez,pkarasev3/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez,mapfau/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,silent-snowman/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez