Video timing simulator: Improve per-frame margin metrics.

This change improves the per-frame margin metrics for the
`RenderingSimulator`:

1) `RenderMargin` is renamed to `AssembledMargin`, to better describe
   the purpose: difference between the target render timestamp and
   the assembly timestamp. This margin can be both positive, zero,
   and negative.
2) A new `RenderedMargin` is added, which is the differerence between
   the target render timestamp and the actual render timestamp. This
   margin should be ==0 most of the time, and <0 whenever an
   underrun happens.

In summary, the new `AssembledMargin` will describe dynamics _before_
the frame buffering, and the new `RenderedMargin` will describe dynamics
_after_ the frame buffering.

Other smaller changes: a) fix bug in render timestamp, b) add total
buffer duration helper, c) add "in time" bool helpers.

Bug: b/423646186
Change-Id: I78e0e2434b4771e35713520a5842f074d3b8c84c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/447840
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46868}