Start backing up wallpaper

This CL does the following:
   + adds an AbsoluteFileBackupHelper class for managing backup of files
     known by absolute path, not based off of the app's getFilesDir() root
   + bumps up the collection interval from its testing-only default of 1 second
     to 3 minutes
   + adds a SystemBackupAgent class to the main system package and names it as
     the android:backupAgent for the main OS package.  Right now this agent
     only backs up & restores the wallpaper file.
   + amend the Wallpaper Service to inform the Backup Manager when the wallpaper
     changes.

On the subject of the 3-minute collection interval before the backup actually
occurs:  this can be short-circuited from an adb shell.  Running the command
'bmgr run' will cause the Backup Manager to kick off any pending backup
operations immediately.