Handle reordered packets and random loss in ScreamV2

The loss event rate is updated on RTT boundaries (at least one max_rtt
since the last filter update). If one or more packets are reported lost
during an RTT interval, the EWMA filter treats the interval as a loss
event.
To handle packet reordering robustly, the estimator maintains a count of
outstanding unrecovered lost packets. When a packet is reported lost for
the first time, the outstanding count increments. If a subsequently
received feedback report indicates that an out-of-order packet has been
recovered, the count decrements. If the unrecovered count reaches zero,
the connection is considered loss-free, and the loss event rate is reset
to zero.
The loss event rate is used for deciding if the last loss event should should trigger reference window backoff.

Bug: webrtc:447037083
Change-Id: I109228a9d51be13dac621f2ac81e16597efb7f9e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/471940
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47732}