Move MediaTransportInterface out of the libjingle_peerconnection_api target

And move related files into api/transport/ and api/transport/media/.
The moved files are unchanged, except that
congestion_control_interface.h and datagram_transport_interface.h
no longer include media_transport_interface.h, instead, they forward
declare the few MediaTransport* types they reference.

Bug: webrtc:8733
Change-Id: I4f4000d0d111f10d15a54c99af27ec26c46ae652
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152482
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Bjorn Mellem <mellem@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29178}
diff --git a/media/base/media_channel.h b/media/base/media_channel.h
index 9cc7876..da4f0d2 100644
--- a/media/base/media_channel.h
+++ b/media/base/media_channel.h
@@ -22,9 +22,9 @@
 #include "api/audio_options.h"
 #include "api/crypto/frame_decryptor_interface.h"
 #include "api/crypto/frame_encryptor_interface.h"
-#include "api/media_transport_config.h"
 #include "api/rtc_error.h"
 #include "api/rtp_parameters.h"
+#include "api/transport/media/media_transport_config.h"
 #include "api/transport/rtp/rtp_source.h"
 #include "api/video/video_content_type.h"
 #include "api/video/video_sink_interface.h"