Shell SizeChanged event not invoked / Size not set

### Description

The `Microsoft.Maui.Controls.Shell.SizeChanged` even inherited from `Microsoft.Maui.Controls.VisualElement.SizeChanged` is not invoked after resizing the window.

Possibly due to `Shell.Width` and `Shell.Height` not being set. (they remain at `-1`)

### Steps to Reproduce

1. `File` -> `New` -> `Project...`
2. Select `A project for creating .NET MAUI application for iOS, Android, Mac, Catalyst, Tizen and WinUI.` -> `Next` -> `Create`
3. Open `AppShell.xaml.cs` and paste:
```c#
namespace MauiAppBugOnSizeChange;

public partial class AppShell : Shell
{
public AppShell()
{
InitializeComponent();

SizeChanged += AppShell_SizeChanged;
}

private void AppShell_SizeChanged(object sender, EventArgs e)
{
System.Diagnostics.Debug.WriteLine("Shell size changed!");
System.Diagnostics.Debug.WriteLine($"Width: {this.Width}\t WidthRequest: {this.WidthRequest}");
}
}
```
4. (Optional) Add a breakpoint inside the `AppShell_SizeChanged()` function.
5. `F5 Start Debuging` (Windows Machine)
6. Resize the application window

### Version with bug

Release Candidate 3 (current)

### Last version that worked well

Unknown/Other

### Affected platforms

Windows

### Affected platform versions

Windows 21H2 (OS Build 19044.1645)

### Did you find any workaround?

The `SizeChanged` event in `ContentPage` does work.
I've tried linking this event to the `Shell` object's event as follows
```c#
private void MainPage_SizeChanged(object sender, EventArgs e)
{
try
{
AppShell shell = (AppShell)Application.Current.MainPage;
shell.AppShell_SizeChanged(sender, e);
}
catch (NullReferenceException) { }
}
```

While this does _sort of_ work it it wasn't a solution to my use case. (#7161)
It did however give a possible explanation as to why this event is not fired.

The `Width` and `WidthRequest` properties of a `Shell` object remain at `-1`.
Thus @jsuarezruiz implementation https://github.com/dotnet/maui/commit/250021582ca9e08395f0d7fd61f0a80525f85259 / #4541 isn't processed.

### Relevant log output

_No response_

Start with the AppShell.xaml.cs reproduction on a Windows Machine and compare Shell.SizeChanged with the working ContentPage.SizeChanged behavior. Inspect the Shell sizing path related to #4541 and commit 250021582ca9e08395f0d7fd61f0a80525f85259; done means resizing invokes the Shell event and updates its Width and Height.

Written by the indexing model from the issue text.