Enable dynamic speed controller by default, with new AV1 defaults.

This CL enables real-time adaptive changes of encoder speeds, reacting
to encoded quality encode time constraints. Only libaom AV1 should be
affected at this time as the speed controller is not yet wired in in
any other encoder implementation. The defaults are set to "high" for
camera content and "low" for screencast, pending potential future
experimentation.

Bug: webrtc:443906251
Change-Id: Id7cee9428faf9db05193f6437c373426ed7c2c86
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475460
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47820}