Remove using AudioManager.FLAG_ACTIVE_MEDIA_ONLY

FLAG_ACTIVE_MEDIA_ONLY is not used inside Audio Framework code, but
is only needed internally for MediaSessionService. Replace it with
boolean musicOnly value.

Bug: 160682430
Test: Manually test log when trying to change volume while no media
is playing

Change-Id: I6a1dee04e7aab0d7602096a257e0375e32d795dd