Reland "Simplification and refactoring of the AudioBuffer code"

This is a reland of 81c0cf287c8514cb1cd6f3baca484d668c6eb128

Original change's description:
> Simplification and refactoring of the AudioBuffer code
> 
> This CL performs a major refactoring and simplification
> of the AudioBuffer code that.
> -Removes 7 of the 9 internal buffers of the AudioBuffer.
> -Avoids the implicit copying required to keep the
>  internal buffers in sync.
> -Removes all code relating to handling of fixed-point
>  sample data in the AudioBuffer.
> -Changes the naming of the class methods to reflect
>  that only floating point is handled.
> -Corrects some bugs in the code.
> -Extends the handling of internal downmixing to be
>  more generic.
> 
> Bug: webrtc:10882
> Change-Id: I12c8af156fbe366b154744a0a1b3d926bf7be572
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149828
> Commit-Queue: Per Åhgren <peah@webrtc.org>
> Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28928}

Bug: webrtc:10882
Change-Id: I2ddf327e80a03468c41662ae63c619ff34f2363a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150101
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28938}
diff --git a/modules/audio_processing/noise_suppression_impl.cc b/modules/audio_processing/noise_suppression_impl.cc
index c834717..151af61 100644
--- a/modules/audio_processing/noise_suppression_impl.cc
+++ b/modules/audio_processing/noise_suppression_impl.cc
@@ -82,7 +82,7 @@
   RTC_DCHECK_EQ(suppressors_.size(), audio->num_channels());
   for (size_t i = 0; i < suppressors_.size(); i++) {
     WebRtcNs_Analyze(suppressors_[i]->state(),
-                     audio->split_bands_const_f(i)[kBand0To8kHz]);
+                     audio->split_bands_const(i)[kBand0To8kHz]);
   }
 #endif
 }
@@ -98,19 +98,19 @@
   RTC_DCHECK_EQ(suppressors_.size(), audio->num_channels());
   for (size_t i = 0; i < suppressors_.size(); i++) {
 #if defined(WEBRTC_NS_FLOAT)
-    WebRtcNs_Process(suppressors_[i]->state(), audio->split_bands_const_f(i),
-                     audio->num_bands(), audio->split_bands_f(i));
+    WebRtcNs_Process(suppressors_[i]->state(), audio->split_bands_const(i),
+                     audio->num_bands(), audio->split_bands(i));
 #elif defined(WEBRTC_NS_FIXED)
     int16_t split_band_data[AudioBuffer::kMaxNumBands]
                            [AudioBuffer::kMaxSplitFrameLength];
     int16_t* split_bands[AudioBuffer::kMaxNumBands] = {
         split_band_data[0], split_band_data[1], split_band_data[2]};
-    audio->CopySplitChannelDataTo(i, split_bands);
+    audio->ExportSplitChannelData(i, split_bands);
 
     WebRtcNsx_Process(suppressors_[i]->state(), split_bands, audio->num_bands(),
                       split_bands);
 
-    audio->CopySplitChannelDataFrom(i, split_bands);
+    audio->ImportSplitChannelData(i, split_bands);
 #endif
   }
 }