Update quality convergence only on base temporal layer frames.

When temporal layering is used, VideoStreamEncoder::OnEncodedImage()
passes the QP of every encoded frame (T0, T1, T2) to
QualityConvergenceController::AddSampleAndCheckTargetQuality() and
FrameCadenceAdapterInterface::UpdateLayerQualityConvergence() [1].
Non-reference T>0 frames do not update the T0 reference buffer, so a T>0
frame reaching static_qp_threshold [2] prematurely halts ZeroHertz
convergence repeats before the T0 reference buffer converges, and
interleaving T0 and T>0 QPs skews the 12-sample dynamic QP window [3].

Gate quality convergence updates in VideoStreamEncoder::OnEncodedImage()
on TemporalIndex().value_or(0) == 0. Guard this by the
enabled-by-default WebRTC-QCM-IgnoreEnhancementTemporalLayers
kill-switch.

[1]
https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/video/video_stream_encoder.cc;l=2317-2325
[2]
https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/video/quality_convergence_monitor.cc;l=146-150
[3]
https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/video/quality_convergence_monitor.cc;l=166-193

Bug: chromium:328598314
Change-Id: I383b5e2aa69463513e474861151788681e7a6ff5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/509742
Reviewed-by: Johannes Kron <kron@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48970}