Implement ANA statistics.

This CL also makes it possible to enable/prevent ANA controllers from making adaptations using field trials.

BUG=webrtc:8127

Review-Url: https://codereview.webrtc.org/3007983002
Cr-Original-Commit-Position: refs/heads/master@{#19761}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 17289097f08280b377c4ca550e8d75e46ffcede3