Auto offscreen-composite elevated descendants under reduced opacity on Android (flagged) - #58493

…n Android (flagged)

Summary:
On Android, a view drawn with reduced `opacity` that contains a descendant with
`elevation` composites per drawing primitive, so the elevation shadow renders as
concentric bands with an over-opaque center instead of fading uniformly (see
react#23090).

Behind the new default-off feature flag
`enableAndroidAutoOffscreenCompositingForElevation`, `ReactViewGroup` reports
overlapping rendering (`hasOverlappingRendering()` returns true) whenever it
contains a descendant with `elevation`, so the framework bakes the offscreen
decision into the RenderNode at record time. Any alpha applied afterwards --
static `opacity`, a JS-driven animation, or a native-driver opacity animation
that sets alpha directly on the RenderNode without re-recording -- then
composites the subtree through that layer and the shadow fades uniformly. A layer
is only actually allocated when `alpha < 1`, so this costs nothing at full
opacity. This is the automatic equivalent of the community
`needsOffscreenAlphaCompositing` workaround.

The descendant-elevation check is cached and lazily invalidated: it is marked
stale from the logical mount/unmount and elevation hooks -- via a new `uimanager`
interface `ElevatedDescendantCache`, never from the subview-clipping/scroll path
-- and rescanned once on the next query. So the per-draw callback stays O(1),
mounting N children stays O(N) rather than O(N^2), and scrolling a
`removeClippedSubviews` list does no per-frame work. The cache is self-correcting
and cannot drift like a maintained counter. Behavior is unchanged unless the flag
is enabled.

Adds ReactViewGroupTest coverage: flag on/off, alpha == 1 (layer readied ahead of
a later alpha change), no / nested elevated descendant, an elevated view inside a
non-ReactViewGroup container, invalidation bubbling through a non-ReactViewGroup
ancestor, an elevated child added after a prior query, removal, and
needsOffscreenAlphaCompositing.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D118581751