Fix Android MVCP anchor selection when z-index reorders children

[hsource]

Summary:

Motivation

At Wanderlog, we use a FlatList with negative zIndex so earlier items' bottom shadows can cover later items (by default, later items cover earlier items).

When maintainVisibleContentPosition is enabled on Android, computeTargetView selects the scroll anchor by scanning content children and picking the first one whose end position exceeds the current scroll offset. Fabric can reorder native children when views use zIndex, so hierarchy order no longer matches layout order. In that case the helper can anchor to the wrong item, and scroll corrections keep the bottom edge of the visible content fixed instead of the top.

Fix

This change scans all eligible children and selects the topmost visible anchor (the smallest end position still below the scroll offset). It also clears the cached anchor when the helper stops listening.

Ported from the approach in #46247, adapted for the current Kotlin helper, and simplified.

Changelog:

[ANDROID] [FIXED] - Fix maintainVisibleContentPosition anchoring to the wrong child when z-index reorders Fabric children

Test Plan:

Manual test on Android using RNTester:

1. Open ScrollView → smooth bi-directional content loading
2. Add a few items and scroll so there are items above and below the current item
3. Use Change height at id to change the height of an item in the middle
4. Verify the top edge of that item stays in place, rather than the bottom edge jumping

Before (maintained position relative to bottom)	After (correctly maintains position relative to top)

[hsource]

This change was made because cloneElement seemed to remove the zIndex, so we instead render items without cloneElement.

While I was at it, I also changed this to a function component.