Move the stereo-disables-CNG logic from CodecManager to Rent-A-Codec

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1473563004

Cr-Original-Commit-Position: refs/heads/master@{#10784}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 989b4abcf32d4a1605e7af160a1a07916c6b324e