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/test/audio_processing_simulator.cc b/modules/audio_processing/test/audio_processing_simulator.cc
index 8c6a46d..7f354a9 100644
--- a/modules/audio_processing/test/audio_processing_simulator.cc
+++ b/modules/audio_processing/test/audio_processing_simulator.cc
@@ -504,6 +504,12 @@
         *settings_.maximum_internal_processing_rate;
   }
 
+  const bool use_legacy_ns =
+      settings_.use_legacy_ns && *settings_.use_legacy_ns;
+  if (use_legacy_ns) {
+    apm_config.noise_suppression.use_legacy_ns = use_legacy_ns;
+  }
+
   if (settings_.use_ns) {
     apm_config.noise_suppression.enabled = *settings_.use_ns;
   }
diff --git a/modules/audio_processing/test/audio_processing_simulator.h b/modules/audio_processing/test/audio_processing_simulator.h
index 340c9d3..bf718b2 100644
--- a/modules/audio_processing/test/audio_processing_simulator.h
+++ b/modules/audio_processing/test/audio_processing_simulator.h
@@ -65,6 +65,7 @@
   absl::optional<bool> use_extended_filter;
   absl::optional<bool> use_drift_compensation;
   absl::optional<bool> use_legacy_aec;
+  absl::optional<bool> use_legacy_ns;
   absl::optional<bool> use_experimental_agc;
   absl::optional<bool> use_experimental_agc_agc2_level_estimator;
   absl::optional<bool> experimental_agc_disable_digital_adaptive;
diff --git a/modules/audio_processing/test/audioproc_float_impl.cc b/modules/audio_processing/test/audioproc_float_impl.cc
index 6428e9d..3e755b5 100644
--- a/modules/audio_processing/test/audioproc_float_impl.cc
+++ b/modules/audio_processing/test/audioproc_float_impl.cc
@@ -130,6 +130,10 @@
           kParameterNotSpecifiedValue,
           "Activate (1) or deactivate(0) the legacy AEC");
 ABSL_FLAG(int,
+          use_legacy_ns,
+          kParameterNotSpecifiedValue,
+          "Activate (1) or deactivate(0) the legacy AEC");
+ABSL_FLAG(int,
           experimental_agc,
           kParameterNotSpecifiedValue,
           "Activate (1) or deactivate(0) the experimental AGC");
@@ -393,6 +397,8 @@
 
   SetSettingIfFlagSet(absl::GetFlag(FLAGS_use_legacy_aec),
                       &settings.use_legacy_aec);
+  SetSettingIfFlagSet(absl::GetFlag(FLAGS_use_legacy_ns),
+                      &settings.use_legacy_ns);
   SetSettingIfFlagSet(absl::GetFlag(FLAGS_experimental_agc),
                       &settings.use_experimental_agc);
   SetSettingIfFlagSet(