Deferring wallpaper update to improve workspace scrolling (issue 5506959)

-> On the Xoom, this change gets us back up to 60 fps. The
   change is really more of a workaround for the fact that we don't
   have vsync, and we ought to be able to change it back once we do.

Change-Id: I80888f18887bf5f2fed72c19641ed430ef6dbfcf