Add Lock volume keys in silent mode [2/2]

Prevents silent mode from accidentally being unset by volume keys

Forward ported from GB

PS 2 : Reorder

Change-Id: Ia86d191ac4db1e369fd3f9ac25644eb51feab67e

Conflicts:
	res/xml/sound_settings_rom.xml
	src/com/android/settings/slim/SoundSettings.java

Conflicts:
	res/xml/sound_settings.xml
	src/com/android/settings/SoundSettings.java