Move Call::receive_time_calculator_ to network thread

This CL moves the usage of receive_time_calculator_ in Call from the
worker thread to the network thread. It extracts the worker thread logic
of DeliverRtpPacket into DeliverRtpPacket_w to allow clearer thread
annotations.

Also includes new g3doc for a simple overview of the RTP receive flow.

Bug: webrtc:11993
Change-Id: I143a548888474b4ebd14a9041d5cb649738e0775
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/454304
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47119}