VCS: Minor fix for initial volume set and refactor

Fix initial volume set, check connected devices from group instead of
all connected devices to update volume when user-set volume flag is set.

Allow to use setDeviceVolume without existing of LeAudioService
when called without GroupOp.

Simplify handleGroupNodeAdded.

Change wrong error log severity in VolumeControlStateMachine.

Bug: 373377157
Test: atest VolumeControlServiceTest VolumeControlStateMachineTest
Flag: EXEMPT; Regression covered with unit test

Change-Id: If70ffa596d0af841b4d80a810003a135b6d7342f