Move AudioDecoderIsac* to its own files

webrtc/modules/audio_coding/codecs/isac/fix/interface/audio_decoder_isacfix.h

+/*
+ *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_MODULES_AUDIO_CODING_CODECS_ISAC_FIX_INTERFACE_AUDIO_DECODER_ISACFIX_H_
+#define WEBRTC_MODULES_AUDIO_CODING_CODECS_ISAC_FIX_INTERFACE_AUDIO_DECODER_ISACFIX_H_
+
+#include "webrtc/modules/audio_coding/codecs/isac/audio_decoder_isac_t.h"
+#include "webrtc/modules/audio_coding/codecs/isac/fix/source/isac_fix_type.h"
+
+namespace webrtc {
+
+using AudioDecoderIsacFix = AudioDecoderIsacT<IsacFix>;
+
+}  // namespace webrtc
+#endif  // WEBRTC_MODULES_AUDIO_CODING_CODECS_ISAC_FIX_INTERFACE_AUDIO_DECODER_ISACFIX_H_