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/media/engine/webrtcmediaengine.cc b/webrtc/media/engine/webrtcmediaengine.cc
index 1a82839..9b5fef2 100644
--- a/webrtc/media/engine/webrtcmediaengine.cc
+++ b/webrtc/media/engine/webrtcmediaengine.cc
@@ -9,16 +9,17 @@
  */
 
 #include "webrtc/media/engine/webrtcmediaengine.h"
-#include "webrtc/modules/audio_coding/codecs/builtin_audio_decoder_factory.h"
 
 #include <algorithm>
 
+#include "webrtc/api/audio_codecs/builtin_audio_decoder_factory.h"
+#include "webrtc/media/engine/webrtcvoiceengine.h"
+
 #ifdef HAVE_WEBRTC_VIDEO
 #include "webrtc/media/engine/webrtcvideoengine2.h"
 #else
 #include "webrtc/media/engine/nullwebrtcvideoengine.h"
 #endif
-#include "webrtc/media/engine/webrtcvoiceengine.h"
 
 namespace cricket {