Reify the transport lookup/selection API

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