AGC2 adaptive digital controller config clean-up

- Remove dry-run option
- Hard-code `adjacent_speech_frames_threshold` and
  `vad_reset_period_ms`
- Expose `initial_gain_db` via field trial

Tested: adaptive digital controller bit-exactness verified

Bug: webrtc:7494
Change-Id: I6166611f91320b6c37de3f8e553c06c2ed95b772
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/287222
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Hanna Silen <silen@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38862}
diff --git a/modules/audio_processing/audio_processing_impl.h b/modules/audio_processing/audio_processing_impl.h
index 189ed03..0f74c30 100644
--- a/modules/audio_processing/audio_processing_impl.h
+++ b/modules/audio_processing/audio_processing_impl.h
@@ -142,12 +142,8 @@
   // removed.
   struct GainController2ConfigOverride {
     InputVolumeController::Config input_volume_controller_config;
-    struct AdaptiveDigitalConfig {
-      float headroom_db;
-      float max_gain_db;
-      float max_gain_change_db_per_second;
-      float max_output_noise_level_dbfs;
-    } adaptive_digital_config;
+    AudioProcessing::Config::GainController2::AdaptiveDigital
+        adaptive_digital_config;
   };
 
  protected: