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.cc

@@ -167,6 +167,10 @@ void PacketBuffer::ForceSpsPpsIdrIsH264Keyframe() {
   sps_pps_idr_is_h264_keyframe_ = true;
 }
 
+void PacketBuffer::ResetSpsPpsIdrIsH264Keyframe() {
+  sps_pps_idr_is_h264_keyframe_ = false;
+}
+
 void PacketBuffer::ClearInternal() {
   for (auto& entry : buffer_) {
     entry = nullptr;