Move signaling safety flag into SctpDataChannel and clarify its purpose

Encapsulate the signaling thread safety flag within SctpDataChannel and
rename to `controller_safety` to clarify its purpose. Remove details
about the flag from the CreateProxy() interface. The observer previously
received this flag via the CreateProxy method, but the observer is
associated with the channel (not the proxy). Basically, the proxy
creation layer should not need to know about the flag mechanism and the
channel object was out of the loop, so this was a bit awkward.

Bug: webrtc:510487699
Change-Id: I57638ade52305b6479a9a9165747336c679631cd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/304841
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47649}