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

Change-Id: Iea7a2eed86d4b65d3772b95f613d1853c66a0158
Signed-off-by: anerik <anerikgames@gmail.com>

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