Halo: Add master switch to disable Halo (1/2)

Change-Id: Ifb0f4e8abdce1ed3d4cc4c973edc08794781db64

Conflicts:
	core/java/android/provider/Settings.java
	packages/SystemUI/src/com/android/systemui/statusbar/BaseStatusBar.java

Conflicts:

	core/java/android/provider/Settings.java