[lib] Update thick thread avatars logic

Summary:
We should correctly select thick thread avatar based on user avatars.

https://linear.app/comm/issue/ENG-9374/thread-avatars-not-using-user-avatars-for-dms

Test Plan: Created personal and private thick threads and verified that on a thread list their avatars are correct, and before this diff they were incorrect.

Reviewers: kamil, will

Reviewed By: will

Differential Revision: https://phab.comm.dev/D13462