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/audio/BUILD.gn b/audio/BUILD.gn
index 654e810..078d6ab 100644
--- a/audio/BUILD.gn
+++ b/audio/BUILD.gn
@@ -67,9 +67,9 @@
     "../logging:rtc_event_log_api",
     "../logging:rtc_stream_config",
     "../modules/audio_coding",
+    "../modules/audio_coding:audio_encoder_cng",
     "../modules/audio_coding:audio_format_conversion",
     "../modules/audio_coding:audio_network_adaptor_config",
-    "../modules/audio_coding:cng",
     "../modules/audio_device",
     "../modules/audio_processing",
     "../modules/bitrate_controller:bitrate_controller",