Update encoder settings periodically, not only on new bandwidth estimate

Also moved actual update call to encoder thread, and tweaked frame rate
estimate to be less noisy.

This is a re-upload of https://review.webrtc.org/47249004

BUG=chromium:476469
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1180623005.

Cr-Commit-Position: refs/heads/master@{#9417}
5 files changed