[lib] Pass in threadInfos into getChatMentionCandidates

Summary:
Small diff to just pass in threadInfos into getChatMentionCandidates, since the next diff will use it. The next diff has a lot of changes to deal with Flow, so I thought separating this small change into its own diff made sense. If the reviewers disagree, I'm happy to merge this with the diff that uses threadInfos.

Part of ENG-5436

Depends on D10458

Test Plan: Ran flow again since there are no real changes to test here

Reviewers: atul, ginsu, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10459