a2dp: return NotSupported error if no server found
a2dp: return NotSupported error if no server found

With both sink and sources disabled, the A2DP server is not even
registered in audio_manager_init. When trying to register the endpoint,
this should result in the same error as if the server existed but the
profile was disabled.

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