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/test/fuzzers/audio_decoder_fuzzer.cc b/webrtc/test/fuzzers/audio_decoder_fuzzer.cc
index 54c56ad..adc0fc7 100644
--- a/webrtc/test/fuzzers/audio_decoder_fuzzer.cc
+++ b/webrtc/test/fuzzers/audio_decoder_fuzzer.cc
@@ -12,9 +12,9 @@
 
 #include <limits>
 
+#include "webrtc/api/audio_codecs/audio_decoder.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/base/optional.h"
-#include "webrtc/modules/audio_coding/codecs/audio_decoder.h"
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 
 namespace webrtc {