APM: rename `AnalogGainStatsReporter` to `InputVolumeStatsReporter`

Adopt the new naming convention, which replaces "analog gain" and
"mic level" with "input volume", in the input volume stats reporter.

Bug: webrtc:7494
Change-Id: Ia24876151f51dd1dcc4e4f9db56c64d11ae3b442
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/279840
Reviewed-by: Hanna Silen <silen@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38467}
diff --git a/modules/audio_processing/audio_processing_impl.h b/modules/audio_processing/audio_processing_impl.h
index 3dfa8ce..bc582b5 100644
--- a/modules/audio_processing/audio_processing_impl.h
+++ b/modules/audio_processing/audio_processing_impl.h
@@ -539,7 +539,7 @@
   RmsLevel capture_output_rms_ RTC_GUARDED_BY(mutex_capture_);
   int capture_rms_interval_counter_ RTC_GUARDED_BY(mutex_capture_) = 0;
 
-  AnalogGainStatsReporter input_volume_stats_reporter_
+  InputVolumeStatsReporter applied_input_volume_stats_reporter_
       RTC_GUARDED_BY(mutex_capture_);
 
   // Lock protection not needed.