Previously, GroupDescription.Internal served only as a marker for
internal groups and simply delegated to AccountGroup. By converting
GroupDescription.Internal to a real interface, we gain a lot of
flexibility. For instance, we are free to use another class to
represent internal groups (which will be necessary for the migration
of groups to NoteDb).
Change-Id: If7397898d8508184a2ccdc7c371ed60ada7d6f3e