media: Add HFP/HSP gains to transport API
media: Add HFP/HSP gains to transport API

Add two independent and HSP/HFP-specific properties to the media
transport representing the input and output audio volumes.

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