API CHANGE: Backup/restore API changes requested by the API Council

* @hide the android.app.backup.RestoreSession class and functionality

* Provide a public method on android.app.backup.BackupManager that apps
  can use to request a restore pass of their last-known-good dataset.
  The new method is called requestRestore().

* Provide the name of the package being restored, not just its ordinal,
  in the RestoreObserver's onUpdate() callback.

Part of bug #2545514

Change-Id: I9689bf8d6e2b808b4ee412424a36a835be0a5ca8