[lib] Fix memoization of useRelationshipPrompt

Summary:
We were using callbacks but the return value was never memoized.
Depends on D3734

Test Plan: Send friend request by using relationship prompt and check if the state was updated.

Reviewers: benschac, atul, def-au1t

Reviewed By: atul

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3735