VolumeControl: rm impossible null handler

Use final & injected dependency
Update test to TestLooper & truth
Split test to actionable smaller test
Apply up to date java recommendation

Bug: 368527278
Bug: 367133232
Test: atest BluetoothInstrumentationTests:VolumeControlServiceTest
Test: atest BluetoothInstrumentationTests:VolumeControlStateMachineTest
Flag: Exempt refactor no-op
Change-Id: I15a9122a10abbd4ed598a1af5b8a8902967b9135