Make RenderUiKitView reject absorbed touch events (#28666)

When a touch event that is in the bounds of a RenderUiKitView is absorbed by another render object,
the RenderUiKitView's handleEvent is not called for that object. On the platform side, the touch event hits the FlutterTouchInterceptingView which is waiting for a framework decision that never arrived on whether to reject or accept the gesture.

This change fixes the issue by having RenderUiKitView register a global PointerRoute, that is used to reject absorbed touch events.