Updated DASH-IF livesim asset URLs and added new ones

[TobbeEdgeware]

Changed livesim assets to new host URLs https://livesim.dashif.org.
Added SegmentTimeline with $Number$ in addition to SegmentTimeline with $Time$.
Addes startOver URLS: interval [-20s, 20s] compared to wall-clock. These turn into static at interval end.
Corrected some information.

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[TobbeEdgeware]

I've fixed the corporate CLA now.

[googlebot]

CLAs look good, thanks!

[joeyparrish]

Looks good to me. Thank you for the PR!

[shaka-bot]

All tests passed!

[joeyparrish]

PR cherry-picked for v2.4.6.