[SCReAM] Self-contain Application-Limited detection via fixed-window acked rate

Previously, SCReAM v2 did not autonomously monitor the actual received (acked) bitrate on the channel to evaluate if a session was truly Application-Limited.

With this CL, ScreamV2 tracks the received (acked) transfer rate over a fixed 100ms window and evaluates is_application_limited internally by checking if the received rate is less than 0.9 times the target rate. When periodic padding or probing bursts are transmitted, the receive rate scales up close to the target rate and automatically unfreezes the reference window so BWE can adapt upwards to full link capacity. In addition, leaving ALR when the receive rate is close to the target rate ensures that RTT and feedback hold time estimates are updated faster.

In addition, ReceivedRateWindow and AlrThreshold parameters have been exposed via field trials in ScreamV2Parameters to allow runtime configuration.

Bug: webrtc:447037083
Change-Id: I1ba1a6f098944e79431e46a8547149f2b084c610
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/482680
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48034}