Rename new_content_prekeys and new_notif_prekeys

Summary:
We only upload one of each type of prekey, so these fields should not be plural

Depends on D12436

Test Plan: ran flow, checked all invocations of RefreshUserPrekeys to make sure client code reflected the field name changes

Reviewers: bartek, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12441