[lib] Memoize array that gets passed to useThreadSearchIndex

Summary: This addresses ENG-4605 by fixing the regression that I introduced here.

Test Plan: I was able to repro the issue in my local environment by opening the SidebarListModal on my iOS simulator. After this diff, the issue no longer reproed

Reviewers: atul, tomek, ginsu

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D8760