[lib] Don't send activity updates for pending threads
Summary:
ENG-4393
After activating id migration, activityUpdateResponder started throwing errors, because the conversion logic didn't expect pending ids. This diff stops clients from sending updates for pending thread. They were already ignored by the keyserver anyway, so there should be no behaviour changes on the keyserver.
Test Plan:
- Create a new pending thread
- Without this diff there are invalid_client_id_prefix in the keyserver console. After this diff there are no error.
- Checked that activity updates for other threads are still sent
Reviewers: kamil, inka, tomek
Reviewed By: tomek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D8495