Stop iOS SetNeedsLayout propagation by looking at VisualElement computed Constraint

[albyrock87]

Description of Change

#26629 removed SetNeedsLayout propagation via inheritance, this was needed to fix #24996.
Unfortunately, this also removed the ability to intercept and stop propagation as needed.

This PR enhances what we've done in the previous one by giving the stop-propagation control back to the platform view by returning a boolean on bool IPlatformMeasureInvalidationController.InvalidateMeasure(bool isPropagating).

MauiView now looks at the computed internal LayoutConstraint Constraint => ComputedConstraint | SelfConstraint; property to see whether the view has fixed constraints.
This is achieved by checking whether we're propagating and CrossPlatformLayout is IConstrainedView { HasFixedConstraints: true }.

Page now implements that interface with a fixed => true so I've been able to remove the hardcoded stop propagation logic if (superview is ContentView { IsPage: true } or UIScrollView) completely.

This fixes the attached issue because it allows (unfortunately only via reflection for now), to set VisualElement.SelfConstraint to Fixed = 3 on a node to stop the propagation.

That said this automatically solves use cases where the user is setting WidthRequest and HeightRequest to a fixed value.

Issues Fixed

Fixes #28410

[dotnet-policy-service]

Hey there @albyrock87! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.