VCS: Add wrappers for getGroupId and getGroupDevices

Add wrappers for getGroupId and getGroupDevices to simplify code and
ensure consistent data retrieval from the CSIP or LeAudioService.

Read groups from CSIP first as in phone policy it is connected first.
LeAudioService get group IDs from it if available.

Improve some unit tests by use inOrder.
Use service instead of binder if test is not focused on it.
Add to unit test generateVolumeStateChanged after connection as it
always happen in current code base.
Add new unit tests for wrappers.

Bug: 373377157
Flag: EXEMPT; Refactor covered with unit tests
Test: atest VolumeControlServiceTest
Change-Id: Ia131e035d2e2681c0ed52e6760f6bfd94ec3de77