Start tracking the container count per policy including reporting
it in account HEAD and supporting installations where the DB
existed before the updated schema.
Migration is triggered by the account audtior; if the database is
un-migrated it will continue to report policy_stats without the per
policy container_count keys.
Closes-Bug: #1367514
Change-Id: I07331cea177e19b3df303609a4ac510765a19162