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/modules/audio_coding/neteq/decoder_database.h b/webrtc/modules/audio_coding/neteq/decoder_database.h
index 83789da..114cca3 100644
--- a/webrtc/modules/audio_coding/neteq/decoder_database.h
+++ b/webrtc/modules/audio_coding/neteq/decoder_database.h
@@ -15,10 +15,10 @@
 #include <memory>
 #include <string>
 
+#include "webrtc/api/audio_codecs/audio_decoder_factory.h"
+#include "webrtc/api/audio_codecs/audio_format.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/common_types.h"  // NULL
-#include "webrtc/modules/audio_coding/codecs/audio_decoder_factory.h"
-#include "webrtc/modules/audio_coding/codecs/audio_format.h"
 #include "webrtc/modules/audio_coding/codecs/cng/webrtc_cng.h"
 #include "webrtc/modules/audio_coding/neteq/audio_decoder_impl.h"
 #include "webrtc/modules/audio_coding/neteq/packet.h"