audio: Add Volume property to A2DP transport GetProperties
audio: Add Volume property to A2DP transport GetProperties

Now that volume is being handled by SetProperty it should also be
available in GetProperties.

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