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

Change-Id: Iea7a2eed86d4b65d3772b95f613d1853c66a0158
Wallpaper Overscroll and Transparent NavBar

Now possible to offset wallpaper windows by more than the size of the wallpaper
and add added a transparent mode to the NavigationBar

Change-Id: I2b0f1129a4cf73b8def35abb7072d0472a9cc972

Conflicts:
	core/java/android/app/WallpaperManager.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java
	services/java/com/android/server/wm/Session.java
	services/java/com/android/server/wm/WindowManagerService.java
	services/java/com/android/server/wm/WindowState.java