[lib] Narrow threadUIName to ThreadInfo

Summary:
Part of https://linear.app/comm/issue/ENG-6366/rename-minimallyencodedrawthreadinfo-to-rawthreadinfo

I obviously don't want to make a separate diff for every function I modify in thread-utils, but figured it made sense to make this its own diff since there are corresponding changes to thread-settings-modal at callsite.

This also removes a bit of branching.

Depends on D10645

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10646