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.
