Update AEC3 echo tail estimation.

Note: estimation is turned OFF if config_.ep_strength.default_len
is set >= 0 (in this case config_.ep_strength.default_len defines a
constant echo decay factor), and hence turned ON if < 0. In case the
echo tail estimation is turned ON, -config_.ep_strength.default_len is
the starting point for the estimator.

The estimation is done in two passes; first we go through all "sections"
(corresponding to chunks of length kFftLengthBy2) of the filter impulse
response to determine which sections correspond to a "stable" decay",
and then the second pass we go through each stable decay section and
estimate the decay. The actual decay estimation is based on linear
regression of the log magnitude of the squared impulse response.
A bunch of sanity checks are also performed continuously to avoid
estimation error during e.g., filter adaptation.

Bug: webrtc:8924
Change-Id: I686ce3f3e8b6b472348f8d6e01fb44c31e25145d
Reviewed-on: https://webrtc-review.googlesource.com/48440
Commit-Queue: Christian Schuldt <cschuldt@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22247}

modules/audio_processing/include/audio_processing.h

@@ -1162,6 +1162,7 @@ class VoiceDetection {
  protected:
   virtual ~VoiceDetection() {}
 };
+
 }  // namespace webrtc
 
 #endif  // MODULES_AUDIO_PROCESSING_INCLUDE_AUDIO_PROCESSING_H_