Remove all AudioBuffer code that is not related to storing audio data

This CL moves/removes all code from the AudioBuffer that:
-Is not directly handling audio data (e.g., keytaps, VAD descisions).
-Is caching aggregated versions of the rest of the audio data.
-Is not used (or only used in testing)

Bug: webrtc:10882
Change-Id: I737deb3f692748eff30f46ad806b2c6f6292802c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149072
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28866}

modules/audio_processing/echo_control_mobile_impl.h

@@ -54,6 +54,7 @@ class EchoControlMobileImpl {
 
   void ProcessRenderAudio(rtc::ArrayView<const int16_t> packed_render_audio);
   int ProcessCaptureAudio(AudioBuffer* audio, int stream_delay_ms);
+  void CopyLowPassReference(AudioBuffer* audio);
 
   void Initialize(int sample_rate_hz,
                   size_t num_reverse_channels,
@@ -78,6 +79,8 @@ class EchoControlMobileImpl {
 
   std::vector<std::unique_ptr<Canceller>> cancellers_;
   std::unique_ptr<StreamProperties> stream_properties_;
+  std::array<std::array<int16_t, 160>, 2> low_pass_reference_;
+  bool reference_copied_ = false;
 };
 }  // namespace webrtc