Remove getChangeIntersectionEntry from LegacyAdIntersectionObserverHost

[samouri]

summary
Replaces the usage of custom-element.getChangeIntersectionEntry with an actual IntersectionObserver. For the fire() call, uses an unobserve/observe pair.

Note: I think we could remove batching since InOb will handle that automatically (a series of many observe/unobserve) pairs in rapid succession will only emit a single event)

Partial for #31540

[samouri]

pushed an empty commit just to rerun owners check. otherwise will merge when it passes

[Tudmotu]

Hi all,
We started getting null rootBounds objects in 3p iframe (amp-ad). Was there some backward-incompatible change here?

[jridgewell]

@Tudmotu See #32087, we're currently deploying the fix. Should be live today.

[Tudmotu]

Thank you 🙏