[native] Prevent the only admin role from being changed

Summary:
Just like on web, we want to make sure the last remaining admin in a community cannot remove themselves from the role until someone else is assigned the Admins role

Depends on D8160

Test Plan:
Please see the video below that demonstrates this

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: ted, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8161