Move AudioDecoder and related stuff to the api/ directory

BUG=webrtc:5805, webrtc:6725

Review-Url: https://codereview.webrtc.org/2668523004
Cr-Commit-Position: refs/heads/master@{#16534}
diff --git a/webrtc/api/BUILD.gn b/webrtc/api/BUILD.gn
index 8c443cb..2e3d5fe 100644
--- a/webrtc/api/BUILD.gn
+++ b/webrtc/api/BUILD.gn
@@ -29,8 +29,8 @@
     ":transport_api",
     "..:webrtc_common",
     "../base:rtc_base_approved",
-    "../modules/audio_coding:audio_decoder_factory_interface",
     "../modules/audio_coding:audio_encoder_interface",
+    "audio_codecs:audio_codecs_api",
   ]
 }