[lib] Make sure we set isViewer in useExistingThreadInfoFinder

Summary:
When we call createPendingThread in useExistingThreadInfoFinder, we need to include isViewer in order to make sure a PRIVATE thread is rendered as eg. "ashoat" instead of "just you". This is because the naming is based on ifJustViewer, which only applies if the members list only includes the viewer.

Depends on D13546

Test Plan: Confirm that I see eg. "ashoat" instead of "just you" for PRIVATE thread uiName

Reviewers: tomek, kamil, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13547