SCReAMv2: Replace EWMA loss rate filter with asymmetric step filter

Replaces the complex EWMA loss rate filter in SCReAMv2 with an intuitive
and highly robust Asymmetric Step Filter (leaky bucket counter) to estimate
the short-term congestion level of the connection.

The new filter tracks short-term congestion on RTT boundaries and is normalized
between 0.0 (loss-free) and 1.0 (congested):
- Increment step (loss RTT): 1.0 / rtts_with_loss_before_backoff (default 3).
  This ensures exactly 3 consecutive loss RTTs are required to trigger backoff from a clean state.
- Decrement step (lossless RTT): 1.0 / lossless_rtts_before_clear (default 2).
  This ensures the filter is fully cleared back to 0.0 after exactly 2 consecutive lossless RTTs.

The expected drift under 1% uniform random loss is strongly negative (-0.5),
guaranteeing spurious wireless link losses are filtered out, while a hardcoded
congested() helper check protects against floating-point rounding errors across
various tuned parameter values.

Additional Cleanups:
- Renamed LossRateEstimator class and files to LossEstimator.
- Renamed loss_event_rate() getter and variables to congestion_level() and congestion_level_.
- Simplified scream_v2.cc window growth by checking congestion_level() == 0.0 directly,
  removing the redundant loss_event_rate_threshold_increase and loss_event_rate_threshold_discard
  parameters.
- Updated all unittests and verified that all integration tests pass.

Bug: webrtc:447037083
Change-Id: Iaf1ac08a5230f6e72af3cb0ed84423dfb6997e39
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475580
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47839}