Move towards a formal public API for backup and restore

This commit makes a few changes towards establishing a formal application
interface for interacting with the backup/restore mechanism:

1. Introduce public wrapper classes around the various binder interfaces; 3rd
party code will never see the binders directly.

2. Progress update callbacks during a restore sequence now occur on the main
thread, not in a binder thread [and not with system-process permissions!].

3. Rename the BackupManagerService's inner "RestoreSession" class to avoid
ambiguity with the new public "RestoreSession" class.