Option to use volume keys to control media volume anytime (1/2)

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.

Change-Id: I6dbd25d1a14bcf5b1bf0b0468b96f4b4f8efdb14

Conflicts:
	res/layout/preference_dialog_ringervolume.xml
	res/values/cm_strings.xml
	src/com/android/settings/RingerVolumePreference.java