[lib] Replace threadSearchIndex selector with useGlobalThreadSearchIndex hook

Summary: Depends on D6477

Test Plan:
There were six surfaces affected by this change:

1. ThreadPickerModal on native
2. ThreadPickerModal on web
3. ChatThreadList on native
4. ThreadListProvider on web (surfaced through ChatThreadList)
5. SidebarsModal on web
6. SubchannelsModal on web

I tested all of them:

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6478