Use standard EWMA with g = 1/16 for SCReAMv2 l4s_alpha

Update ScreamV2::UpdateL4SAlpha to use a single EWMA gain l4s_avg_g = 1/16
(scaled by steps_per_rtt) and interpolate toward fraction_marked on both
increases and decreases:
  l4s_alpha_ = g * fraction_marked + (1 - g) * l4s_alpha_

This aligns with Section 4.2.1.3 of the latest SCReAMv2 editor's draft
(draft-ietf-ccwg-rfc8298bis-screamv2-02) and avoids decaying toward 0
when 0 < fraction_marked <= l4s_alpha_.

Bug: webrtc:447037083
Change-Id: I23e3a66922fea100714bf0ecc6146b9aec416d64
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/509040
Auto-Submit: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48914}