[ios/catalyst] fix more cycles in NavigationPage

[jonathanpeppers]

Fixes: #21453
Context: #22810

In #22810, a leak in NavigationPage was fixed for the case:

Application.Current.MainPage = new NavigationPage(new Page1());
Application.Current.MainPage = new Page2();

However, it does not work for the case:

await Navigation.PushModalAsync(new NavigationPage(new Page1()));
await Navigation.PopModalAsync();

I could reproduce this problem in MemoryTests.cs.

There were still a few cycles in NavigationRenderer:

• NavigationRenderer -> VisualElement _element -> NavigationRenderer

• NavigationRenderer -> Page Current -> NavigationRenderer

• NavigationRenderer -> ViewHandlerDelegator<NavigationPage> _viewHandlerWrapper -> TElement _element -> NavigationRenderer

After fixing these cycles, the test passes. The customer's sample also seems to work:

ViewHandlerDelegator was slightly tricky, as I had to make a _tempElement variable and unset it immediately after use.

[jonathanpeppers]

While SetVirtualView is running, it appears that the getter for Element is called. Best I came up with is to make a _tempElement and clear it similar to:

maui/src/Controls/src/Core/Compatibility/Handlers/VisualElementRenderer.cs

Line 60 in 2c40a29

public TElement? Element => _tempElement ?? (_virtualView is not null && _virtualView.TryGetTarget(out var target) ? target : null);

maui/src/Controls/src/Core/Compatibility/Handlers/VisualElementRenderer.cs

Lines 298 to 304 in 2c40a29

	// _tempElement is used here, because the Element property is accessed before SetVirtualView() returns
	_tempElement = Element;
	SetVirtualView(view, this, OnElementChanged, ref _tempElement, ref _mapper, _defaultMapper, AutoPackage);
	// We use _virtualView as a WeakReference, and clear _tempElement
	_virtualView = _tempElement is null ? null : new(_tempElement);
	_tempElement = null;