get_users_who_share_room_with_user(...) should use the users_who_share_private_rooms/users_in_public_rooms tables instead of calculating it themselves again.

This might be a nice optimization for the presence and device list sharing code where get_users_who_share_room_with_user is used.

Discovered while working on #13958 and seeing code for users sharing rooms in #13966

See #13575 (comment) for the original exploration around finding get_users_in_room mis-uses.