Move AudioDecoderPcm16B next to AudioEncoderPcm16B All AudioDecoder subclasses have historically lived in NetEq, but they fit better with the codec they wrap. BUG=webrtc:4557 R=henrik.lundin@webrtc.org Review URL: https://codereview.webrtc.org/1348113002 . Cr-Original-Commit-Position: refs/heads/master@{#9963} Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc Cr-Mirrored-Commit: c0ac6cad00f51b99fafaa7c19e2e63b3b542735b