Split ConfigureForRtcpFeedback into two setters

PacketRouter::ConfigureForRtcpFeedback set two unrelated things at
once: whether transport sequence numbers are generated for all
packets, and whether packets are marked ECT(1). They have different
triggers. The first only changes when the feedback format is
negotiated, the second whenever an input to the ECT(1) decision
changes.

Combining them means the ECT(1) decision can not be pushed on its own.
A caller that wants to avoid redundant work has to gate both settings
on whether the ECT(1) decision changed, which silently drops transport
sequence number updates. That matters for the upcoming blackhole
detection, which re-evaluates the decision on every sent packet.

No behavior change. SetGenerateTransportSequenceNumbers is now only
called when the feedback format is negotiated, which is the only place
where its input can change.

Bug: webrtc:42225697
Change-Id: I1cb75ee2f2a0d7164652dde6952822c5c6f32244
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/504020
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48633}