[lib] Simplify filterChatThreadList

Summary:
It's good this function got factored out, because it turns out the logic can be significantly simplified now.

Depends on D2885

Test Plan: Flow, careful code inspection

Reviewers: def-au1t, palys-swm

Reviewed By: def-au1t, palys-swm

Subscribers: benschac, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3145