Video timing simulator: Add a `reuse_streams` option to `DecodabilitySimulator`.

This is similar to the changes to `RenderingSimulator` in
https://webrtc-review.googlesource.com/c/src/+/428520. It turns out that
we need this feature for `DecodabilitySimulator` as well, whenever the
latter is used for event log visualization.

Change-Id: I5da84f8565b69de8a5bbe1aa87d3a6db0ee6b96e
Bug: b/423646186
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/442921
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46707}