Further utilizing the AEC3 config struct for constants

BUG=webrtc:5298

Review-Url: https://codereview.webrtc.org/3007833002
Cr-Original-Commit-Position: refs/heads/master@{#19605}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4fed3c0b6f74acbdce2f7c7395463772a19dc739