Add AudioProcessingCaptureStats and a level estimator replacement

This adds an interface for accessing stats on the capture stream, and
adds a level estimator to report one of the stats.

Bug: webrtc:9947
Change-Id: Id472534fa2e04d46c9ab700671f620584a246afb
Reviewed-on: https://webrtc-review.googlesource.com/c/109587
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25786}

modules/audio_processing/include/audio_processing.h

@@ -283,6 +283,11 @@ class AudioProcessing : public rtc::RefCountInterface {
       } adaptive_digital;
     } gain_controller2;
 
+    // Enables reporting of |output_rms_dbfs| in webrtc::AudioProcessingStats.
+    struct LevelEstimation {
+      bool enabled = false;
+    } level_estimation;
+
     // Explicit copy assignment implementation to avoid issues with memory
     // sanitizer complaints in case of self-assignment.
     // TODO(peah): Add buildflag to ensure that this is only included for memory