This is used in multiple places already. In the next diff I introduce a third usage, so I figured it makes sense to factor out.

I put this in a separate file from the component (ListLoadingIndicator) because exporting a constant alongside a React component can break hot reloading (some context here and here).

Depends on D14141

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.