DualPi2: Fix L4S double marking and backlog guard

Aligns DualPi2NetworkQueue marking and dropping behavior with RFC 9332
and Linux sch_dualpi2 (!drop_early):

1. Single L4S marking on dequeue: Previously, ShouldTakeAction() rolled
   the random PI2 marking probability for ECT(1) packets in both
   EnqueuePacket() and DequeuePacket(), resulting in an effective
   marking probability of 2*p - p^2 instead of p. L4S packets are now
   only evaluated for marking on dequeue.
2. Time-based step_threshold: Replaces the byte-based step_threshold_
   (which required Config::link_rate to be set and dropped classic
   packets on enqueue) with Config::step_threshold as a TimeDelta
   (defaulting to PlusInfinity) checked against L4S packet sojourn time
   on dequeue.
3. Standing-queue backlog guard: Adds Config::min_backlog_packets
   (defaulting to 2) so that small 1-packet bursts are not marked or
   dropped when there is no standing queue backlog.

Bug: webrtc:447037083, webrtc:42225697
Change-Id: I2f75a159f895c6b994e2665ce9f77d0fb6f6cb45
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508160
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48841}