AGC2 dummy module: fixed gain param, APM integration, audioproc_f adaptation

webrtc/modules/audio_processing/include/audio_processing.h

Index: webrtc/modules/audio_processing/include/audio_processing.h
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index 03bd8c85b02efff9698b927c7f3f7e1660733a0f..cd70bf06adc1a50cfd32916a968973fd5ec0afe4 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -326,6 +326,8 @@ class AudioProcessing : public rtc::RefCountInterface {
     // does not yet have the desired behavior.
     struct GainController2 {
       bool enabled = false;
+      // Fixed digital gain params.
+      float fixed_gain_db = 0.f;
     } gain_controller2;
 
     // Explicit copy assignment implementation to avoid issues with memory