Status bar Auto-Hide [1/2]

Add an option to auto-hide status bar when there are no
notifications.

PS 2 : Improved Logic

Change-Id: I9f9fdb443f6ed981c1941716491cef757b42f8ba

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