HearingAid profile: rm impossible null handler

Use final & injected dependency
Update test to TestLooper & truth
Split test to actionable smaller test

Fix: 368527278
Bug: 368527278
Test: atest BluetoothInstrumentationTest:HearingAidServiceTest
Test: atest BluetoothInstrumentationTest:HearingAidStateMachineTest
Flag: Exempt refactor
Change-Id: Id5bf8f6d8daf8057b313905e1a13cb993667f2f3