First bit of wallpaper work.

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.

services/java/com/android/server/SystemBackupAgent.java

@@ -53,7 +53,7 @@ public class SystemBackupAgent extends BackupHelperAgent {
         try {
             super.onRestore(data, appVersionCode, newState);
 
-            WallpaperService wallpaper = (WallpaperService)ServiceManager.getService(
+            WallpaperManagerService wallpaper = (WallpaperManagerService)ServiceManager.getService(
                     Context.WALLPAPER_SERVICE);
             wallpaper.settingsRestored();
         } catch (IOException ex) {