HFP client volume control

Accept volume controls from connected phone.  Suport AT+VGS and +VGS
commands.

Bug: 38005564
Test: Connect a phone, start a call, change volume up and down on phone
and verify volume level on headunit changes, as well as monitor snoop
logs.

Change-Id: I8b89b2487da81cf9562598a0a5408c266e8ecc6a