More backup work

* Put in some permission enforcement around agent connection notification
  and full-backup scheduling.
* Full backup now applies to any package, not just backup participants who
  have declared their own android:backupAgent
* The process of running the backup operation on the set of apps who have
  been queued for it is now done in a separate thread, with a notification
  mechanism from the main Backup Manager service to pass along new-agent
  binding knowledge.  There's no longer one do-backup message on the primary
  Handler per target application.
* The new backup thread sets up the desired transport now and passes
  along the newly backed-up data to it for each backup target.  Two
  transports have been defined so far, GoogleTransport and AdbTransport;
  both are stubs at present.

Note that at present the backup data output file seems to be properly
created, but after doBackup() is called on the test app's agent it's
still zero size.