First baby steps towards settings backup

This change adds a sketched outline of the backup system architecture, with
all of the major pieces represented other than client-side helpers for
specific types of data.  IBackupManager and BackupService are public so that
we can write test apps against SDK-domain symbols from the outset.

What code exists in this change hasn't been tested and may crash.  It's the
beginnings of the real implementation but of course is barely begun.