Introduce a stable BackupTransport interface class for transport
implementations to derive from.
Make the interface for viewing/selecting the active backup transport
part of the stable API.
Make restore-related classes (RestoreSession, RestoreSet) stable API.
Express backup manager APIs needed for transport operation as
@SystemApi methods in BackupManager.
Bug 16661321
Change-Id: I423b87ae8f45c1b77831d4f8ffd97715484c2d2b