[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