Make RestoreSession.getAvailableRestoreSets() asynchronous

This transaction can involve the transport having to query a remote backend
over the wire, so it can take a Long Time(tm).  Make it main-thread-safe by
making it asynchronous, with the results passed as a callback to the invoker's
RestoreObserver.  We also make the IRestoreObserver callback interface
properly oneway.

Bug #2550665
Bug #2549422

Change-Id: If18a233a0a3d54c7b55101715c9e6195b762c5a0