Wim Taymans authored Oct 24, 2014 and Arun Raghavan committed Oct 31, 2014

Parse the gain changed AT commands from the headset and fire 2 new
hooks as a result. The device will connect to those hooks and change the
source/sink volumes.

When the source/sink volume changes, set the gain on the microphone or
speaker respectively. Make sure we do nothing if the transport can not
handle the gain changes.