Per temporal layer rate control in the V2 libaom AV1 encoder.

The encoder reported every frame as belonging to temporal layer 0, so
libaom ran a single rate control context for the whole stream and sized
every frame against the same budget regardless of how the bitrate was
distributed across the layers. The per frame budget was communicated by
scaling the frame duration, which left the duration meaning something
other than the interval between frames.

Signal the real temporal id instead, declare the layer configuration to
libaom, and derive the cumulative layer bitrates and frame rate factors
it expects via the new TemporalLayerRateTracker. Cbr::duration is now
passed through verbatim, and the meaning of both Cbr fields is spelled
out in video_encoder_interface.h.

The existing rate control tests are adjusted to allow the total to
overshoot by what the keyframes were actually allowed, which the
microsecond precision duration no longer leaves room for.

This is a precursor to
https://webrtc-review.git.corp.google.com/c/src/+/503780

Bug: webrtc:496266459
Change-Id: Ic9183a9757d624293148ec28af0b6adfcd0f2e1c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/504280
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48659}