Inline UITraitChangeObservable in implementing classes

[rolfbjarne]

This is a follow-up to #19410.

We need to inline the members of the UITraitChangeObservable interface into implementing types (UIView, UIViewController, UIPresentationController, UIWindowScene). This turned out a bit cumbersome to implement, because C# doesn't support calling the default interface member from an actual implementing member (#13294 (comment)).

Here's the current WIP: rolfbjarne@5e09575 / https://github.com/rolfbjarne/xamarin-macios/commits/uitraitchangeobservable-inlined-edition

This is probably much easier to do once we've added support for default interface members (#13294), so postpone until then.