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_unittest.cc b/webrtc/media/engine/webrtcmediaengine_unittest.cc
index 03e50a1..c58c75b 100644
--- a/webrtc/media/engine/webrtcmediaengine_unittest.cc
+++ b/webrtc/media/engine/webrtcmediaengine_unittest.cc
@@ -8,8 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include "webrtc/api/audio_codecs/builtin_audio_decoder_factory.h"
 #include "webrtc/media/engine/webrtcmediaengine.h"
-#include "webrtc/modules/audio_coding/codecs/builtin_audio_decoder_factory.h"
 #include "webrtc/test/gtest.h"
 
 using webrtc::RtpExtension;