The main consumer of this feature is shortcuts.
The LOOKUP KEY column will contain an encoded concatenation of the contact's row id
as well as sync_ids of all constituent raw contacts. It goes with the "contacts/lookup/*/#" URI.
When we get such a URI, we will first try to load the
contact with the specified _id and lookup_key. If we cannot find the contact
that way, we will go scout for the contact that contains most of the sync_ids
we found in the lookup key.
We will need to make sure that the contact picker returns the lookup-style URIs.
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