[lib] Don't construct promises in useCreateChatThreadItem

Summary:
This is the first diff in a stack that reduces the creation of Promises, which seems to overwhelm Hermes (see here).

This diff overhauls useCreateChatThreadItem:

• It converts getLastUpdatedTime so that it returns a function that returns a Promise instead of just a Promise.
• It replaces several of the returned properties with a single getFinalChatThreadItem() async function that returns all of them. I think this is cleaner.
• It introduces lastUpdatedAtMostTimeIncludingSidebars. We need this for a later diff, which uses it to guess an order of which ChatThreadItems to check in order to get the top N ChatThreadItems by (resolved) lastUpdatedTimeIncludingSidebars. This way, we avoid creating the promises unless we need them.

Depends on D13984

Test Plan: I tested this task by playing around with the ChatThreadList on mobile while using a stopwatch to measure how long various operations took. I tested the updated code 3 times against both the most recent Testflight build as well as master. I found that performance was approximately the same as before. I tested scrolling down, selecting threads that were read, selecting threads that were unread, and search.

Reviewers: tomek, angelika

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D14136