Backport "Option to use volume keys to control media volume anytime" from CM10.1

Some users don't adjust ringtone volume often (e.g. only use toggle to
switch between silent and non-silent) mode. Having an option to
use the volume keys to control media volume anytime allows media
volume to be controllled/muted before entering a game or other apps
with sound in an undesirable location.

Originally author: Pawit Pornkitprasan
Backport from CM10.1 by: Daveee10