[lib][native] Review usingOlmViaTunnelbrokerForDMs and pendingThreadType

Summary:
issue: ENG-9146
pendingThreadType should actually return thick or thin types depending on where we call it from.
We want to keep the usingOlmViaTunnelbrokerForDMs flag, so that for now thick threads are not created.

Test Plan: Tested that with the flag set to true thick threads can be created. Checked that with the flag set to false, thin threads are created instead.

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13258