[lib] Rename threadInfos to resolvedThreadInfos in getChatMentionCandidates

Summary:
Just a quick rename diff to make it clear that the thread infos passed in are resolved. I'm doing this because the following diffs will involve passing in the unresolved ThreadInfos as a parameter alongside the resolved thread infos.

Part of ENG-5436

Test Plan: Ran flow to confirm no errors

Reviewers: atul, ginsu, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10458