Allow signaled SSRCs to override learned bindings in RtpDemuxer

Update RtpDemuxer to permit signaled SSRCs to override existing learned
SSRC bindings. This prevents registration failures during session
renegotiation when an SSRC, previously identified through payload type
demuxing, is subsequently explicitly signaled.

The demuxer now maintains a set of signaled SSRCs to differentiate
explicit configurations from learned state. Conflict detection is
modified to allow these overrides while maintaining strict checks
between multiple signaled SSRCs. This ensures that explicit signaling
always takes precedence over internally learned bindings, resolving
routing issues during media stream updates.

Bug: webrtc:502130956
Change-Id: I551c2b7ee5af18fc9792ee52be08c5349c20b641
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/464220
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47446}