A2dpSink: mainThread for device stateMachine

Add Looper injection support to be able to have a TestLooper in the test
and control the code execution. This will remove any flakiness due to
timing issues

Bug: 298281348
Test: atest A2dpSinkServiceTest
Test: atest A2dpSinkStateMachineTest
Test: atest BluetoothInstrumentationTests
Change-Id: I9d662db25f343e4414f7e097c1baf11da36697e2