[lib] Temporarily switch LOCAL to COMMUNITY_SECRET_SUBTHREAD for pending threads

Summary:
These two constants were hardcoded to be the same value. I used LOCAL instead of COMMUNITY_SECRET_SUBTHREAD in this case as a reminder to the author that we should be created thick threads here instead of thin threads eventually.

I'm ready to introduce a separate constant for LOCAL, but we'll need to do some additional work before we're able to switch to using it for new pending threads. For now I'll move this code to continue to use the same constant (COMMUNITY_SECRET_SUBTHREAD, which will stay as 4), but I'll return it back to using threadTypes.LOCAL once we're ready.

Depends on D12486

Test Plan: Flow

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12487