Fixes: dotnet#13520
Context: https://github.com/danardelean/MauiTestFinalizer

In the above sample, you can see `Page` objects leaking while using
Shell navigation. I was able to reproduce this in a test with both
Shell & `NavigationPage` such as:

    [Fact(DisplayName = "NavigationPage Does Not Leak")]
    public async Task DoesNotLeak()
    {
        SetupBuilder();
        WeakReference pageReference = null;
        var navPage = new NavigationPage(new ContentPage { Title = "Page 1" });

        await CreateHandlerAndAddToWindow<WindowHandlerStub>(new Window(navPage), async (handler) =>
        {
            var page = new ContentPage { Title = "Page 2" };
            pageReference = new WeakReference(page);
            await navPage.Navigation.PushAsync(page);
            await navPage.Navigation.PopAsync();
        });

        await Task.Yield();
        GC.Collect();
        GC.WaitForPendingFinalizers();
        Assert.NotNull(pageReference);
        Assert.False(pageReference.IsAlive, "Page should not be alive!");
    }

To summarize:

* Customer's sample only leaks on iOS/Catalyst

* My Shell/NavigationPage tests leak on Android/iOS/Catalyst

Comparing various GC heap dumps in Visual Studio, I was able to find
the sources of these issues. I'll try to list them.

* Controls.Core
    * `Window.OnPageChanged()` should unset `_menuBarTracker.Target`
* Windows
    * `ContentViewHandler.DisconnectHandler` should unset
      `platformView.CrossPlatformMeasure` and
      `platformView.CrossPlatformArrange`.
* Android
    * `ContentViewHandler.DisconnectHandler` should unset
      `platformView.CrossPlatformMeasure` and
      `platformView.CrossPlatformArrange`.
    * `NavigationViewFragment.OnDestroy()` should unset `_currentView`
      and `_fragmentContainerView`
* iOS/Catalyst
    * `ContentViewHandler.DisconnectHandler` should unset
      `platformView.CrossPlatformMeasure` and
      `platformView.CrossPlatformArrange`.
    * `ContainerViewController`0 should call `ClearSubviews()` on its
      view in `WillMoveToParentViewController()`
    * `ParentingViewController` should forward `WillMoveToParentViewController()`
      to its child view controllers

After these changes, my tests pass! They literally would not pass
without fixing all of the above... Each issue I found while reviewing
GC dumps.

Other test changes:

* `ControlsHandlerTestBase.Windows.cs` needed some `null` checks
* `ShellTests` needed an `&& !MACCATALYST` to build for MacCatalyst
* Added tests proving we can reuse pages still.