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: I4ef1f720f58785d98989a7e31c424498432cee1d
Merge changes I4ef1f720,Iea7a2eed into du44

* changes:
  Wallpaper Overscroll and Transparent NavBar
  Fix calculating alpha for back button in landscape mode.