Battery Profile: rm impossible null handler

Use final & injected dependency
Update test to TestLooper & truth
Split test to actionable smaller test
Force test to use disconnecting state

Bug: 367133232
Fix: 367133232
Test: atest BluetoothInstrumentationTests:BatteryServiceTest
Test: atest BluetoothInstrumentationTests:BatteryStateMachineTest
Flag: Exempt refactor no-op
Change-Id: Ia0187a77934262303dbf34d2f63f3cc64dc1ecaa