Heads up master switch

Create a global heads up switch to enable/disable the same.
The user still need to set each app in Settings -> Apps

PS: disabling through master switch will automatically turn of  call ui and mms heads up notifications

Change-Id: I650e230d275247251435c5cfeb728e375e927045

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