Simplify JsepIceCandidate construction and state

This is in preparation of reducing use of Candidate in the PC interface
in favor of IceCandidate. This also facilitates changes to the Candidate
class itself and removal of use of the transport_name() attribute.

Using No-try since there's currently an infra problem and all runs
passed green a few hours ago (no code changes since then).

No-try: true
No-Iwyu: grpc_signaling.cc tool needs unrelated dependency updates.
Bug: webrtc:406795492, webrtc:8395
Change-Id: I9059867baeb91345fb61fa4e990bcca42e1417af
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/393923
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#44805}