Clean up decoders when stopping video receive stream.

This updates VideoReceiveStream2::Stop() to symmetrically tear down
state that's built up in VideoReceiveStream2::Start().

Bug: webrtc:11993, webrtc:14486
Change-Id: I41f4feea5584e5baaeed2143432136f8b9761321
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272537
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38244}

modules/video_coding/packet_buffer.h

@@ -82,6 +82,7 @@ class PacketBuffer {
   void Clear();
 
   void ForceSpsPpsIdrIsH264Keyframe();
+  void ResetSpsPpsIdrIsH264Keyframe();
 
  private:
   void ClearInternal();