Reland "Move signaling safety flag into SctpDataChannel and clarify its purpose"

This is a reland of commit 81d438a4fb11bc0de35229bd3894db6712935319

The reland captures the controller flag in the ObserverAdapter directly
rather than accessing it via the channel object.

Original change's description:
> 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}

Bug: webrtc:510487699
Change-Id: I7dd7db0f76047d9f741d8280e0adfb4206fdbd5b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/471140
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47669}