[lib] Show viewer's username as thread name for PRIVATE threads if no thread name specified

Summary:
We should treat PRIVATE the same as GENESIS_PRIVATE here.

Depends on D13545

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/D13546