SystemUI: fix calculating alpha for back button in landscape mode.

Change-Id: Iea7a2eed86d4b65d3772b95f613d1853c66a0158

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
Keyguard: smooth background change.

Change-Id: Ia0e4d0000db72b5cdb6fede03791805e34fa311d