[lib] Introduce usingOlmViaTunnelbrokerForDMs to get DM creation

Summary:
This diff:

1. Introduces usingOlmViaTunnelbrokerForDMs
2. Uses it to gate the ThreadType returned by pendingThreadType
3. Updates createRealThreadFromPendingThread to address Flow types

Actually updating createRealThreadFromPendingThread to support thick thread creation is tracked in ENG-8442.

Depends on D12490

Test Plan: Flow

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12491