Wallpapers: new transitions, hiding when not visible, other cleanup.

This is work on the transitions with wallpapers.  There are now new
animations specifically for leaving the wallpaper and returning to
it, which allow us to have a consistent animation when entering home
and returning to it.  I also renamed the existing animations across
wallpapers, and cleaned up some junk in the various interpolators.

This also now hides the wallpaper surface when it is not visible,
to get rid of the wallpaper flickers people complained about albeit
in a somewhat brutal way. :)  (Though really returning us to the
previous behavior with the same previous bugs and name back to them
not being very visible, yay!)  There is are also some bug fixes
here and there about managing the wallpaper visibility that this
change revealed.

Change-Id: I913990a9a81651728122ed2e1101b75ed2c36fcb