Give Audio{De,En}coderIsac* an "Impl" suffix, to free up the original names

webrtc/test/fuzzers/audio_decoder_isac_incoming_packet_fuzzer.cc

 /*
  *  Copyright (c) 2016 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.
  */
 
 #include "webrtc/modules/audio_coding/codecs/isac/main/include/audio_decoder_isac.h"
 #include "webrtc/test/fuzzers/audio_decoder_fuzzer.h"
 
 namespace webrtc {
 void FuzzOneInput(const uint8_t* data, size_t size) {
-  AudioDecoderIsac dec(16000);
+  AudioDecoderIsacFloatImpl dec(16000);
   FuzzAudioDecoderIncomingPacket(data, size, &dec);
 }
 }  // namespace webrtc