Simple cleanups of AudioDecoder and AudioEncoder classes

* Make sure they're all final and don't allow copying or assignment.

  * Get rid of the single-channel PCM decoder classes.

  * Move some includes from .h to .cc files where possible.

BUG=webrtc:4557

Review URL: https://codereview.webrtc.org/1353803002

Cr-Commit-Position: refs/heads/master@{#10021}

webrtc/modules/audio_coding/codecs/isac/audio_decoder_isac_t.h

@@ -50,4 +50,5 @@ class AudioDecoderIsacT final : public AudioDecoder {
 };
 
 }  // namespace webrtc
+
 #endif  // WEBRTC_MODULES_AUDIO_CODING_CODECS_ISAC_AUDIO_DECODER_ISAC_T_H_

webrtc/modules/audio_coding/codecs/isac/audio_encoder_isac_t.h

@@ -94,4 +94,5 @@ class AudioEncoderIsacT final : public AudioEncoder {
 };
 
 }  // namespace webrtc
+
 #endif  // WEBRTC_MODULES_AUDIO_CODING_CODECS_ISAC_AUDIO_ENCODER_ISAC_T_H_