Frameworks: recreate navbar and statusbar background only when needed means only on theme changes

Change-Id: I36d31e79eeaade29cbad2bfffdc647fd5c846d69

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBarView.java