Use receive clock when calculating receive rate in SCReAM.

Previously, UpdateReceiveRate() used feedback_time (sender's local clock)
to measure elapsed time between feedback reports for received_rate_.
If feedback was delayed or paused, this could lead to inaccurate received
rate calculations.

This change uses the latest received packet's timestamp from the feedback
report to measure the elapsed duration on the remote receive clock.

Bug: webrtc:447037083
Change-Id: I1268363ed1159b49c5ec5f48120c753044e1bbd9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/496781
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48389}