BluetoothVolumeControl: Improve Java API for Volume Control Offset

Add argument for selecting a VOCS instance in setVolumeOffset.
Add instance id argument for onVolumeOffsetChanged callback.
Add callbacks for audio location and audio description change.
Add method to read number of VOCS instances.

Bug: 322779934
Bug: 323156655
Tag: #feature
Test: atest VolumeControlServiceTest BluetoothVolumeControlTest
Change-Id: I080bfb20112ce37c1f6560cd254b4a0c9433b1c1