Hearing Aid Service: set and get activeDevice

* Implement setActiveDevice() and isActiveDevice()
* Add a helper method to notify Settings and Audio about active device
change

Bug: 69623109
Test: instrumentation test and manual test
Change-Id: I3af85a08f648d01974185478ae02def60d467bcb
(cherry picked from commit e80cb5dc3a8bd3478ccddebd14abbb35c7235229)