AEC3 Tuning changes.

This CL adds tuning to AEC3 for the purpose of reducing the impact of
gain changes in the analog microphone gain.

BUG=chromium:710818, webrtc:6018

Review-Url: https://codereview.webrtc.org/2811283003
Cr-Original-Commit-Position: refs/heads/master@{#17673}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 103ac7e7d9c3548bf43ffa3a24b5e89689695c02