Improve H26xPacketBuffer management in RtpVideoStreamReceiver2

Address issues with the lifecycle and configuration of H26xPacketBuffer
when codecs are added, updated, or removed mid-stream. Previously, the
receiver could leak keyframe settings across codec changes, ignore
payload type parameter updates, or allocate unnecessary packet buffers.

- Reset the keyframe configuration state to false when removing codecs.
- Use insert_or_assign to ensure payload type and codec parameter
  updates apply correctly on re-registration.
- Update the H.264 keyframe requirement setting on the existing buffer
  in-place to avoid recreating it mid-stream and dropping packets.
- Only allocate the H26xPacketBuffer if an H26x codec is configured,
  either on receive startup or when adding a codec while active.

Bug: webrtc:41480904
Change-Id: Ic3d4b54c6f8e6881f38ba37c784f186008355e24
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/494740
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48313}