More work on wallpapers: animations, lifecycle, scaling, etc.

Yet more work on improving the behavior of wallpapers.  This fixes a few
problems in their lifecycle (corresponding change in the picker also
required for this), makes their animations better for hardware that supports
alpha fades, adds animations for the wallpapers themselves, eliminates
fixed size wallpapers, and adjusts the API for retrieving a wallpaper
bitmap to take care of scaling the raw wallpaper image to match the current
desired width and height.

Change-Id: If1c0aaceba4ea4e175dcb7a8416ca7ddbb9bfa6f