[lib] update creatorID -> unread in CreateThickRawThreadInfoInput

Summary:
ENG-8568

We need to use CreateThickRawThreadInfoInput when creating ADD_VIEWER_TO_THREAD_MEMBERS, where it's complicated to retrieve creatorID.

creatorID is used only for computing unread, that being said we can simplify this.

Test Plan: Flow

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13181