Move congestion controller to a separate module.

This allows other projects to more easily depend on this.

The plan is to move remote_bitrate_estimator and bitrate_controller into this module and reduce the exposed interface to only a simplified version of congestion_controller.h.

No functional changes in this CL.

R=mflodman@webrtc.org, pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1718473002 .

Cr-Commit-Position: refs/heads/master@{#11718}

webrtc/call/BUILD.gn

@@ -12,7 +12,6 @@ source_set("call") {
   sources = [
     "bitrate_allocator.cc",
     "call.cc",
-    "congestion_controller.cc",
     "transport_adapter.cc",
     "transport_adapter.h",
   ]
@@ -29,6 +28,7 @@ source_set("call") {
   deps = [
     "..:rtc_event_log",
     "..:webrtc_common",
+    "../modules/congestion_controller",
     "../modules/rtp_rtcp",
     "../system_wrappers",
   ]