floss: Implement AVRCP volume interface

 - Fill in VolumeInterfaceImpl implementation.
 - Add AVRCP callback disptach

BUG: 194854406
Tag: #floss
Test: Manual test with audio server changes &&
dbus-send --system  --type=method_call \
--print-reply --dest=org.chromium.bluetooth \
/org/chromium/bluetooth/hci0/media \
org.chromium.bluetooth.BluetoothMedia.SetVolume  int32:100
to verify the headset volume is changed

Change-Id: I753095effd4d2235c6d47e8453b90455a4eb467e