[native] Don't render all threads in ChatThreadList

Summary: I noticed Android ChatThreadList performance was really bad for my user, and it boiled down to rendering too many items in the FlatList.

Test Plan:

1. Test to make sure performance issue is fixed
2. Test to make sure new items get loaded seamlessly
3. Test the search experience and how the new reset-count-and-scroll-to-top-when-search-text-changed behavior works
4. Confirm the count gets reset when navigating away from the ChatThreadList

Reviewers: palys-swm, KatPo

Subscribers: Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1090