Change echo detector to scoped_refptr

The echo detector is currently stored as a unique_ptr, but when injecting an echo detector, a scoped_refptr makes more sense since the ownership will be shared.

Bug: webrtc:8732
Change-Id: I2180014acb84f1cd5c361864a444b7b6574520f5
Reviewed-on: https://webrtc-review.googlesource.com/83325
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23610}

modules/audio_processing/audio_processing_impl.h

@@ -44,7 +44,7 @@ class AudioProcessingImpl : public AudioProcessing {
                       std::unique_ptr<CustomProcessing> capture_post_processor,
                       std::unique_ptr<CustomProcessing> render_pre_processor,
                       std::unique_ptr<EchoControlFactory> echo_control_factory,
-                      std::unique_ptr<EchoDetector> echo_detector,
+                      rtc::scoped_refptr<EchoDetector> echo_detector,
                       NonlinearBeamformer* beamformer);
   ~AudioProcessingImpl() override;
   int Initialize() override;