[native] Add loading indicator at the bottom of ChatThreadList

Summary:
Now that loading is async and can take a bit longer, it's good to have a loading indicator visible to the user.

Depends on D14142

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/D14143