Make inbound-rtp stats object lifetime spec-compliant (behind flag).

When field trial "WebRTC-RTP-Lifetime" is enabled, the creation of the
inbound-rtp stats object is delayed until the first packet is received.
- This aligns with spec and Firefox behavior.

To aid testing, PeerConnectionTestWrapper has new negotiation methods
added to give the test more control without having to write a lot of
boilerplate code.

(A separate CL will deal with outbound-rtp which, while the plan is
they continue to be created before first packet is sent, should be
delayed until after O/A has completed.)

Bug: chromium:406585888
Change-Id: Ibac2128e80e0153659b68cc0f00869e5d1f27a69
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/405740
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45428}