Populate RtcEvent timestamp from propagated clock

Move responsibility of populating the timestamp from RtcEvent default
constructor to RtcEventLog implemtnation(s) because latter has the
access to the propagated clock

Bug: webrtc:42223992
Change-Id: I0f0e62f15a723101a50cbd317692ab0240feb409
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/412300
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45843}