Currently, ReviewDb is still the source of truth for all changes.
Doing a live migration to make NoteDb the source of truth will require
storing some additional metadata in NoteDbChangeState, the first piece
of which is a simple enum value indicating where the primary storage
location is.
For now, since we haven't written any migration code, encountering
this state when reading from the database is a runtime error.
Change-Id: Ia30466a73ac9986449d9dc4cbbbe87f5966724d2