BackupManager wrapper class improvements

+ Now rechecks the cached IBinder each time the wrapper is used, and if it's
still null (i.e. the BackupManager was constructed before the system service
came up) it's refetched.  This lets even system code cache a single
BackupManager instance and just keep making calls through it without worrying
about interactions with the life cycle of the backup service.

+ Added a static dataChanged(packageName) method as a convenience for code that
needs to indicate that some other package needs a backup pass.  This is useful
even for third party code in the case of multiple packages in a shared-uid
situation.