Implemented audio ducking in response to the CAN_DUCK audio focus event
Moved audio focus management into AudioPlayerWithEvents, and moved audio
becoming noisy event to the ExternalMusicControlHandler
Added comments to handle methods

Added test to make sure we don't double register the external control client
Changed variable clazz to desiredClass, to be more descriptive
Merge pull request #154 from theJenix/duck_audio

Audio ducking