Actually set g_id for Contact objects.

Summary:
The g_id field is (presumably) supposed to be the unique ID for contacts coming
from the Google contacts API. But it wasn't being set at all.

Test Plan: Adding tests for contacts code is a work-in-progress.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D9