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}
diff --git a/modules/audio_processing/legacy_ns/defines.h b/modules/audio_processing/legacy_ns/defines.h
new file mode 100644
index 0000000..02e0318
--- /dev/null
+++ b/modules/audio_processing/legacy_ns/defines.h
@@ -0,0 +1,52 @@
+/*
+ *  Copyright (c) 2011 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.
+ */
+
+#ifndef MODULES_AUDIO_PROCESSING_LEGACY_NS_DEFINES_H_
+#define MODULES_AUDIO_PROCESSING_LEGACY_NS_DEFINES_H_
+
+#define BLOCKL_MAX 160        // max processing block length: 160
+#define ANAL_BLOCKL_MAX 256   // max analysis block length: 256
+#define HALF_ANAL_BLOCKL 129  // half max analysis block length + 1
+#define NUM_HIGH_BANDS_MAX 2  // max number of high bands: 2
+
+#define QUANTILE 0.25f
+
+#define SIMULT 3
+#define END_STARTUP_LONG 200
+#define END_STARTUP_SHORT 50
+#define FACTOR 40.f
+#define WIDTH 0.01f
+
+// Length of fft work arrays.
+#define IP_LENGTH \
+  (ANAL_BLOCKL_MAX >> 1)  // must be at least ceil(2 + sqrt(ANAL_BLOCKL_MAX/2))
+#define W_LENGTH (ANAL_BLOCKL_MAX >> 1)
+
+// PARAMETERS FOR NEW METHOD
+#define DD_PR_SNR 0.98f        // DD update of prior SNR
+#define LRT_TAVG 0.5f          // tavg parameter for LRT (previously 0.90)
+#define SPECT_FL_TAVG 0.30f    // tavg parameter for spectral flatness measure
+#define SPECT_DIFF_TAVG 0.30f  // tavg parameter for spectral difference measure
+#define PRIOR_UPDATE 0.1f      // update parameter of prior model
+#define NOISE_UPDATE 0.9f      // update parameter for noise
+#define SPEECH_UPDATE 0.99f    // update parameter when likely speech
+#define WIDTH_PR_MAP 4.0f      // width parameter in sigmoid map for prior model
+#define LRT_FEATURE_THR 0.5f   // default threshold for LRT feature
+#define SF_FEATURE_THR 0.5f  // default threshold for Spectral Flatness feature
+#define SD_FEATURE_THR \
+  0.5f  // default threshold for Spectral Difference feature
+#define PROB_RANGE \
+  0.2f                     // probability threshold for noise state in
+                           // speech/noise likelihood
+#define HIST_PAR_EST 1000  // histogram size for estimation of parameters
+#define GAMMA_PAUSE 0.05f  // update for conservative noise estimate
+//
+#define B_LIM 0.5f  // threshold in final energy gain factor calculation
+#endif              // MODULES_AUDIO_PROCESSING_LEGACY_NS_DEFINES_H_