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
7 files changed