[SCHEMA] Allow contact sync from arbitrary remotes.

Summary:
 * Rename g_id to the more generic uid.
 * Add a 'provider_name' column to the Contact table. This allows us to sync
   contacts from multiple backends for the same account without needing to
   worry about uid collisions between those backends.

Test Plan: Unit test added; Google contact sync manually tested to work.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D72