Fix repeat-frame handling and dropping in LibaomAv1Encoder

Replace the 500 ms RTP timestamp heuristic for repeat frames with
explicit `VideoFrame::RepeatMode` handling when
`WebRTC-LibaomAv1Encoder-DropRepeatFramesOnEnhancementLayers` is
enabled:

- On `RepeatMode::kQualityConvergence`, drop repeat frames scheduled on
  enhancement temporal layers (`tid > 0`) and notify `svc_controller_`
  via `ScalableVideoController::OnFrameDropped()` so subsequent frames
  fall back to valid reference buffers.
- On `RepeatMode::kKeepAlive`, temporarily zero non-TL0 bitrates on
  `svc_controller_` so converged keep-alive frames fast-forward to `T0`,
  and restore the configured `VideoBitrateAllocation` on non-keep-alive
  frames or when a keyframe is requested.
- Disable libaom rate-controller frame dropping (`rc_dropframe_thresh`)
  while encoding `kKeepAlive` repeat frames and restore it on subsequent
  non-keep-alive frames.

Bug: webrtc:445115234
Change-Id: I2125a3976435c9615df6f68174935a4b34b36880
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/515260
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#49237}