Hide the AudioEncoderCng class behind a create function

And put codecs/cng/webrtc_cng.h in a non-public build target while
we're at it.

Bug: webrtc:8396
Change-Id: I9f51dffadfb645cd1454617fad30e09d639ff53c
Reviewed-on: https://webrtc-review.googlesource.com/c/108782
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25486}
diff --git a/modules/audio_coding/test/TestVADDTX.cc b/modules/audio_coding/test/TestVADDTX.cc
index aa91ee0..09c69f9 100644
--- a/modules/audio_coding/test/TestVADDTX.cc
+++ b/modules/audio_coding/test/TestVADDTX.cc
@@ -83,12 +83,12 @@
                                                     absl::nullopt);
   if (vad_mode.has_value() &&
       !absl::EqualsIgnoreCase(codec_format.name, "opus")) {
-    AudioEncoderCng::Config config;
+    AudioEncoderCngConfig config;
     config.speech_encoder = std::move(encoder);
     config.num_channels = 1;
     config.payload_type = cn_payload_type;
     config.vad_mode = vad_mode.value();
-    encoder = absl::make_unique<AudioEncoderCng>(std::move(config));
+    encoder = CreateComfortNoiseEncoder(std::move(config));
     added_comfort_noise = true;
   }
   channel_->SetIsStereo(encoder->NumChannels() > 1);