Option to use volume keys to control media volume anytime [2/2]

The volume keys control the Media volume rather than the Ringer
hence removing the issue of accidentaly changing the Ringer volume.

Credits: Pawitp

Change-Id: Ic0a1c04ae41eac65e9535d6de5f11c0456730315

Conflicts:
	res/values/slim_strings.xml
	res/xml/purity_sound_settings.xml
	src/com/android/settings/purity/SoundSettings.java