bluetooth: Move HSP gain conversions into backend-native

For the upcoming A2DP AVRCP Absolute Volume feature the code in BlueZ5
has to be generic to be reusable. Move this conversion so that it
becomes possible to implement A2DP volume - which uses different values
- on top without duplicating existing callback functionality.
3 jobs for pr/bluetooth-assume-software-volume-for-headset-mic in 4 minutes and 29 seconds (queued for 1 second)