Add Lock volume keys in silent mode [1/2]

Prevents silent mode from accidentally being unset by volume keys

Forward ported from GB

Change-Id: Ic1df89b8e10bc87f7e125441141a73a39aea1474

Conflicts:
	core/java/android/provider/Settings.java
	media/java/android/media/AudioManager.java

Conflicts:
	core/java/android/provider/Settings.java
	media/java/android/media/AudioManager.java