Use Contact URIs instead of contact ids.

Contact ids are not guaranteed to be stable.

Instead use a contact URI (which contains both a lookup key and a
contact id, used only as a hint).

This change is needed for caching, since we do not what to cache the
unstable contact ids.

Bug: 5101753
Change-Id: I34814c8935e706ee898adcedc39c4baeea147d67