Move the ECT(1) marking decision into Ect1Policy

Whether packets are marked ECT(1) was decided in three places in
RtpTransportControllerSend. Collect it in a new Ect1Policy, placed in
modules/congestion_controller since modules can not depend on call.

Deriving the decision from its inputs instead of keeping it as mutable
state fixes two things: ECT(1) is no longer enabled when the congestion
controller does not adapt to ECN, and renegotiation no longer
re-enables it on a route already found to bleach the marking.

SupportsEcnAdaptation() is now read once, when the congestion
controller is created. No controller changes it during a call since
the GoogCC/Scream wrapper was removed.

Bug: webrtc:42225697
Change-Id: If8e88bb2f82ec3e059433a5e206c4438a78ce3f4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/504000
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48635}