[native] Pass whether created community is ANNOUNCEMENT to CommunityCreationMembers

Summary:
We pass announcement: boolean to CommunityCreationMembers so we can conditionally pass COMMUNITY_ANNOUNCEMENT_ROOT or COMMUNITY_ROOT to getPotentialMemberItems(...)

Depends on D8057

Test Plan: Created communities with announcement toggled on/off in CommunityConfig and then logged value in CommunityCreationMembers to ensure it matched up.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8058