ModAudio: option to force music volume control
When enabled, volume keys control music volume instead of ringer volume

Cherry-picked and backported from KitKat branch

Revised by bgcngm:
- adapted to take into consideration the following:
-- checkUpdateRemoteStateIfActive method resides on android.media.AudioService
-- isAfMusicActiveRecently method does not exist, but workaround was arranged
(http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.4_r1/android/media/AudioService.java#2641)