AEC3: Audibility improvements

This CL is created from a work initiated at https://webrtc-review.googlesource.com/c/src/+/61160

The purpose of this work is to improve the performance of the echo canceler (AEC3) when the farend signal contains stationary noises:
- An stationarity estimator of the farend signal has been added for detecting the portions of the farend signal that are pure noise.
- When the echo canceler deals with a portion of the signal that contains basically noise, the echo suppressor is able to back-off and avoid the fading of the nearend speech.

Change-Id: Id4b87fc59f4765bf1fca36d1cab39a49aabe104a
Bug: webrtc:9193,chromium:836790
Reviewed-on: https://webrtc-review.googlesource.com/64141
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Jesus de Vicente Pena <devicentepena@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23024}

modules/audio_processing/aec3/render_buffer.h

@@ -67,6 +67,21 @@ class RenderBuffer {
   // Specifies the recent activity seen in the render signal.
   void SetRenderActivity(bool activity) { render_activity_ = activity; }
 
+  // Returns the headroom between the write and the read positions in the
+  // buffer;
+  int Headroom() const {
+    // The write and read indices are decreased over time.
+    int headroom =
+        fft_buffer_->write < fft_buffer_->read
+            ? fft_buffer_->read - fft_buffer_->write
+            : fft_buffer_->size - fft_buffer_->write + fft_buffer_->read;
+
+    RTC_DCHECK_LE(0, headroom);
+    RTC_DCHECK_GE(fft_buffer_->size, headroom);
+
+    return headroom;
+  }
+
  private:
   const MatrixBuffer* const block_buffer_;
   const VectorBuffer* const spectrum_buffer_;