Prevent SetParameters from overwriting initial encodings

Update RtpSender to only modify initial parameters when the sender
has not yet been assigned an SSRC.

Previously, calling SetParameters would unconditionally overwrite
init_parameters_ with the provided input. On an active sender, this
could lead to inconsistent state during JSEP operations. Specifically,
overwriting these initial values caused issues when rolling back a
description, potentially leading to tracks being incorrectly stopped
or failures when attempting to create new offers after a rollback.

This change ensures that init_parameters_ are only updated via
SetParameters calls when the sender is in a pre-negotiated state
(ssrc_ == 0). This preserves the original configuration integrity
required for correct rollback behavior.

Bug: webrtc:500993975
Change-Id: I15945aa2d2ff73ffa7848eb99350ee9bc28ddf67
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/467120
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47568}