Fix onViewableItemsChanged not firing after StrictMode/Activity remount

[pinpong]

Summary:

Same kind of problem as in #58985.
ViewabilityHelper.dispose() clears the pending minimumViewTime timers but keeps _viewableIndices. onUpdate stores the new indices before it starts the timer, so after a dispose() the helper thinks those items were already handled. Every following onUpdate with the same indices runs into the "nothing changed" early return and onViewableItemsChanged never fires for them. It only gets unstuck once the set of viewable items changes, e.g. by scrolling far enough.

That didn't matter as long as dispose() was only called on an instance that was about to go away. With StrictMode (in dev) and <Activity> the same VirtualizedList instance gets componentWillUnmount and then componentDidMount again, and it keeps its helpers. So if a list gets hidden while a minimumViewTime timer is running, the items that just became viewable are never reported after it's shown again.

The fix is one line, dispose() now also calls resetViewableIndices() so the next update looks at the items again. That can't lead to duplicate callbacks because _onUpdateSync still diffs against _viewableItems and only reports real changes. There's a test for that as well.

One thing this doesn't do: the list doesn't re-run viewability by itself when it comes back. The report happens on the next scroll, cell layout or render batch. I didn't want to change mount behavior in the same PR.

Changelog:

[GENERAL] [FIXED] - Fix onViewableItemsChanged not firing for items whose minimumViewTime timer was cancelled by a StrictMode/Activity remount

Test Plan:

Two new tests in ViewabilityHelper-test.js.

minimumViewTime reports items again after dispose fails on main and passes with the fix:

● onUpdate › minimumViewTime reports items again after dispose

    Expected: 1
    Received: 0

does not report already viewable items again after dispose passes before and after. It's there to make sure the reset doesn't cause duplicate callbacks.

I also tried it with a real VirtualizedList (not part of the PR): viewabilityConfig={{minimumViewTime: 1000, itemVisiblePercentThreshold: 50}}, hide it with <Activity mode="hidden"> while the timer is pending, show it again, send another scroll event. On main onViewableItemsChanged is never called, with the fix it's called once. Same result with the StrictMode double mount when the first update lands before the simulated unmount.

$ yarn jest packages/virtualized-lists
Test Suites: 11 passed, 11 total
Tests:       1 skipped, 204 passed, 205 total
Snapshots:   69 passed, 69 total

$ yarn flow-check
Found 0 errors

eslint and prettier --check are clean for both files.