[lib] Remove parent admin from members list if they are not part of the thread

Summary:
As mentioned in DES-44, there are two changes we want to make to 'Parent admins'. The first (handled in other diffs already), involves
displaying just Admins. The second is handled here, involving no longer showing the parent admin as part of the members list if they are not actually in it.

Depends on D8068

Test Plan:
Confirmed that chats that don't actually involve the local parent admin (user ID 256) will no longer show them in the members list. Chats that do, however, will still show them.

Chat where the parent admin is not a member of the chat
Before:

After:

Chat where the parent admin is a member of the chat
Before:

After (no changes):

Reviewers: atul, ginsu, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8072