Replace CallFactory class with a factory function Bug: webrtc:15574 Change-Id: Ib1d8cff8d7550da3af01693a7bc117a7bd342258 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/330000 Commit-Queue: Danil Chapovalov <danilchap@webrtc.org> Reviewed-by: Harald Alvestrand <hta@webrtc.org> Cr-Commit-Position: refs/heads/main@{#41321}