[native] Memoize construction of sidebars in ChatThreadListItem

Summary:
This was getting re-rendered every time which recursively led to ChatThreadListSidebar, etc getting re-rendered every time. This change means ChatThreadListSidebar will render only once (unless something relevant changes).

Before:

After:

Depends on D9100

Test Plan: Things continue to work as expected.

Reviewers: ginsu, rohan, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9102