leaudio: Improve Volume Control API

On the Le Audio device absolute volume is set via
BluetoothLeAudio.setVolume API. LeAudioService knows which group is the
active group and using BluetoothVolumeControl.setVolumeGroup() API apply
absolute volume.
Each LeAudio device is a part of the group.
In order to support audio balance between e.g. left and right earbud
setVolumeOffset should be use per each device.

This change also allows user to register for the callback which will
be invoked when the volume offset changes on the remote device.

Bug: 150670922
Tag: #feature
Sponsor: @jpawlowski
Test: atest BluetoothInstrumentationTests
Change-Id: I3692e45d285d55d946ae0de187aa1d5261f825b8
Merged-In: I3692e45d285d55d946ae0de187aa1d5261f825b8
(cherry picked from commit 470384a0)