Default streams: don't block media even if on different transceiver.

This fixes some edge cases where early media could cause default
stream that block the actual signaled media from beind delivered.

Bug: webrtc:11477
Change-Id: I8b26df63a690861bd19f083102d1395e882f8733
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183120
Commit-Queue: Taylor <deadbeef@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32030}

pc/test/mock_channel_interface.h

@@ -46,6 +46,7 @@ class MockChannelInterface : public cricket::ChannelInterface {
                webrtc::SdpType,
                std::string*),
               (override));
+  MOCK_METHOD(void, SetPayloadTypeDemuxingEnabled, (bool), (override));
   MOCK_METHOD(const std::vector<StreamParams>&,
               local_streams,
               (),