Remove AudioDeviceObserver and make ADM not inherit from the Module interface.

modules/audio_device/audio_device_impl.h

Index: modules/audio_device/audio_device_impl.h
diff --git a/modules/audio_device/audio_device_impl.h b/modules/audio_device/audio_device_impl.h
index 77c4cc51a2b2ff5d4eea25ee2d4bb04bada5634a..864ab48833f520f9d22088b7717aa4f515009832 100644
--- a/modules/audio_device/audio_device_impl.h
+++ b/modules/audio_device/audio_device_impl.h
@@ -44,15 +44,11 @@ class AudioDeviceModuleImpl : public AudioDeviceModule {
   AudioDeviceModuleImpl(const int32_t id, const AudioLayer audioLayer);
   ~AudioDeviceModuleImpl() override;
 
-  int64_t TimeUntilNextProcess() override;
-  void Process() override;
-
   // Retrieve the currently utilized audio layer
   int32_t ActiveAudioLayer(AudioLayer* audioLayer) const override;
 
   // Error handling
   ErrorCode LastError() const override;
-  int32_t RegisterEventObserver(AudioDeviceObserver* eventCallback) override;
 
   // Full-duplex transportation of PCM audio
   int32_t RegisterAudioCallback(AudioTransport* audioCallback) override;
@@ -178,11 +174,8 @@ class AudioDeviceModuleImpl : public AudioDeviceModule {
   AudioLayer PlatformAudioLayer() const;
 
   rtc::CriticalSection _critSect;
-  rtc::CriticalSection _critSectEventCb;
   rtc::CriticalSection _critSectAudioCb;
 
-  AudioDeviceObserver* _ptrCbAudioDeviceObserver;
-
   AudioDeviceGeneric* _ptrAudioDevice;
 
   AudioDeviceBuffer _audioDeviceBuffer;
@@ -191,7 +184,6 @@ class AudioDeviceModuleImpl : public AudioDeviceModule {
 #endif
   int32_t _id;
   AudioLayer _platformAudioLayer;
-  int64_t _lastProcessTime;
   PlatformType _platformType;
   bool _initialized;
   mutable ErrorCode _lastError;