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/video/BUILD.gn b/video/BUILD.gn
index 8edb069..af34436 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -60,6 +60,7 @@
     "../api:transport_api",
     "../api/rtc_event_log",
     "../api/task_queue",
+    "../api/transport/media:media_transport_interface",
     "../api/video:encoded_image",
     "../api/video:video_bitrate_allocation",
     "../api/video:video_bitrate_allocator",
@@ -266,6 +267,7 @@
       "../api/rtc_event_log:rtc_event_log_factory",
       "../api/task_queue",
       "../api/task_queue:default_task_queue_factory",
+      "../api/transport/media:media_transport_interface",
       "../api/video:builtin_video_bitrate_allocator_factory",
       "../api/video:video_bitrate_allocator_factory",
       "../api/video:video_frame",