Move AGC2 config ToString to the right place and update Validate()

The APM config to string mapping must be in one place (namely,
in `audio_processing.cc`). This CL moves the AGC2 config to string
impl to the right place.

This CL also updates `GainController2::Validate()` and adds the
missing unit tests for the parameters that have recently been added.

Stack buffer size in `AudioProcessing::Config::ToString()` increased
because of the extra params. Syntax near `multi_channel_capture` fixed.
Output string format verified with a JS linter.

Bug: webrtc:7494
Change-Id: I692e1549b7d40c970d88a14c8e83da16325fb54c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187080
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32400}
diff --git a/modules/audio_processing/audio_processing_impl.cc b/modules/audio_processing/audio_processing_impl.cc
index 67208df..3b66ee5 100644
--- a/modules/audio_processing/audio_processing_impl.cc
+++ b/modules/audio_processing/audio_processing_impl.cc
@@ -611,10 +611,8 @@
 
   const bool config_ok = GainController2::Validate(config_.gain_controller2);
   if (!config_ok) {
-    RTC_LOG(LS_ERROR) << "AudioProcessing module config error\n"
-                         "Gain Controller 2: "
-                      << GainController2::ToString(config_.gain_controller2)
-                      << "\nReverting to default parameter set";
+    RTC_LOG(LS_ERROR)
+        << "Invalid Gain Controller 2 config; using the default config.";
     config_.gain_controller2 = AudioProcessing::Config::GainController2();
   }