Making the Analog AGC properly support multi-channel

This CL adds proper multi-channel support to the analog AGC.

Beyond that, it prepares adding multi-channel support to the digital
AGC by removing the tight dependency between the analog and digital
AGC codes.

Bug: webrtc:10859
Change-Id: I4414ccbc3db5dbb5ae069fdf426cbd038375ca7b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159480
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29878}

modules/audio_processing/gain_control_impl.cc

@@ -19,6 +19,7 @@
 #include "modules/audio_processing/logging/apm_data_dumper.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/constructor_magic.h"
+#include "rtc_base/logging.h"
 
 namespace webrtc {
 
@@ -380,6 +381,7 @@ int GainControlImpl::target_level_dbfs() const {
 
 int GainControlImpl::set_compression_gain_db(int gain) {
   if (gain < 0 || gain > 90) {
+    RTC_LOG(LS_ERROR) << "set_compression_gain_db(" << gain << ") failed.";
     return AudioProcessing::kBadParameterError;
   }
   compression_gain_db_ = gain;