Reland "Refactoring of the noise suppressor and adding true multichannel support"

This is a reland of 87a7b82520b83a6cf42da27cdc46142c2eb6248c

Original change's description:
> Refactoring of the noise suppressor and adding true multichannel support
> 
> This CL adds proper multichannel support to the noise suppressor.
> To accomplish that in a safe way, a full refactoring of the noise
> suppressor code has been done.
> 
> Due to floating point precision, the changes made are not entirely
> bitexact. They are, however, very close to being bitexact.
> 
> As a safety measure, the former noise suppressor code is preserved
> and a kill-switch is added to allow revering to the legacy noise
> suppressor in case issues arise.
> 
> Bug: webrtc:10895, b/143344262
> Change-Id: I0b071011b23265ac12e6d4b3956499d122286657
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158407
> Commit-Queue: Per Åhgren <peah@webrtc.org>
> Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29646}

Bug: webrtc:10895, b/143344262
Change-Id: I236f1e67bb0baa4e30908a4cf7a8a7bb55fbced3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158747
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29663}

modules/audio_processing/ns/quantile_noise_estimator.cc

@@ -0,0 +1,88 @@
+/*
+ *  Copyright (c) 2019 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 "modules/audio_processing/ns/quantile_noise_estimator.h"
+
+#include <algorithm>
+
+#include "modules/audio_processing/ns/fast_math.h"
+
+namespace webrtc {
+
+QuantileNoiseEstimator::QuantileNoiseEstimator() {
+  quantile_.fill(0.f);
+  density_.fill(0.3f);
+  log_quantile_.fill(8.f);
+
+  constexpr float kOneBySimult = 1.f / kSimult;
+  for (size_t i = 0; i < kSimult; ++i) {
+    counter_[i] = floor(kLongStartupPhaseBlocks * (i + 1.f) * kOneBySimult);
+  }
+}
+
+void QuantileNoiseEstimator::Estimate(
+    rtc::ArrayView<const float, kFftSizeBy2Plus1> signal_spectrum,
+    rtc::ArrayView<float, kFftSizeBy2Plus1> noise_spectrum) {
+  std::array<float, kFftSizeBy2Plus1> log_spectrum;
+  LogApproximation(signal_spectrum, log_spectrum);
+
+  int quantile_index_to_return = -1;
+  // Loop over simultaneous estimates.
+  for (int s = 0, k = 0; s < kSimult;
+       ++s, k += static_cast<int>(kFftSizeBy2Plus1)) {
+    const float one_by_counter_plus_1 = 1.f / (counter_[s] + 1.f);
+    for (int i = 0, j = k; i < static_cast<int>(kFftSizeBy2Plus1); ++i, ++j) {
+      // Update log quantile estimate.
+      const float delta = density_[j] > 1.f ? 40.f / density_[j] : 40.f;
+
+      const float multiplier = delta * one_by_counter_plus_1;
+      if (log_spectrum[i] > log_quantile_[j]) {
+        log_quantile_[j] += 0.25f * multiplier;
+      } else {
+        log_quantile_[j] -= 0.75f * multiplier;
+      }
+
+      // Update density estimate.
+      constexpr float kWidth = 0.01f;
+      constexpr float kOneByWidthPlus2 = 1.f / (2.f * kWidth);
+      if (fabs(log_spectrum[i] - log_quantile_[j]) < kWidth) {
+        density_[j] = (counter_[s] * density_[j] + kOneByWidthPlus2) *
+                      one_by_counter_plus_1;
+      }
+    }
+
+    if (counter_[s] >= kLongStartupPhaseBlocks) {
+      counter_[s] = 0;
+      if (num_updates_ >= kLongStartupPhaseBlocks) {
+        quantile_index_to_return = k;
+      }
+    }
+
+    ++counter_[s];
+  }
+
+  // Sequentially update the noise during startup.
+  if (num_updates_ < kLongStartupPhaseBlocks) {
+    // Use the last "s" to get noise during startup that differ from zero.
+    quantile_index_to_return = kFftSizeBy2Plus1 * (kSimult - 1);
+    ++num_updates_;
+  }
+
+  if (quantile_index_to_return >= 0) {
+    ExpApproximation(
+        rtc::ArrayView<const float>(&log_quantile_[quantile_index_to_return],
+                                    kFftSizeBy2Plus1),
+        quantile_);
+  }
+
+  std::copy(quantile_.begin(), quantile_.end(), noise_spectrum.begin());
+}
+
+}  // namespace webrtc