Move AudioDecoderPcm* next to AudioEncoderPcm*

All AudioDecoder subclasses have historically lived in NetEq, but they
fit better with the codec they wrap.

BUG=webrtc:4557

Review URL: https://codereview.webrtc.org/1348613003

Cr-Original-Commit-Position: refs/heads/master@{#10015}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6faf5bebbab70fd358364b901e36c236bbe84046
8 files changed