This is all of the basic pieces:
- The WallpaperService now creates a surface with the window manager for its
contents.
- There is a simple service that displays a bitmap.
- The wallpaper manager takes care of starting and stopping the service.
- The window manager knows about wallpaper windows and how to layer them with
the windows that want to be shown on top of wallpaper.
Lots and lots of issues remain, but at this point you can actually write a
wallpaper service, select it in the UI, and see it behind an activity.
This is mostly refactoring, adding a new WallpaperManager class that takes care
of the old wallpaper APIs on Context, so we don't need to pollute Context with
various new wallpaper APIs as they are needed. Also adds the first little
definition of a wallpaper service, which is not yet used or useful.
