[lib] Including pending thread for viewer in useThreadListSearch

Summary:
Before this work, we always had a GENESIS_PRIVATE thread for the viewer, so we had no reason to support pending private threads in useThreadListSearch.

Now we would like to support that. We'll show a pending private thread only if no private thread exists.

Depends on D13536

Test Plan: In combination with the rest of the stack, test creating a PRIVATE thread via thread list search on both web and native

Reviewers: tomek, kamil, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13537