Surface ResidualEchoDetector creation to API

This allows users to inject the residual echo detector, as a step toward making it an optional part of compilation.

Bug: webrtc:11292, webrtc:11539
Change-Id: I7fcc8dbaced67a82851cd6cdcbc115eb01c21fcf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174040
Reviewed-by: Per Åhgren <peah@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31222}

api/audio/echo_detector_creator.cc

@@ -0,0 +1,21 @@
+/*
+ *  Copyright (c) 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#include "api/audio/echo_detector_creator.h"
+
+#include "modules/audio_processing/residual_echo_detector.h"
+#include "rtc_base/ref_counted_object.h"
+
+namespace webrtc {
+
+rtc::scoped_refptr<EchoDetector> CreateEchoDetector() {
+  return new rtc::RefCountedObject<ResidualEchoDetector>();
+}
+
+}  // namespace webrtc