When app overrides "TextControlCommandBarContextFlyout" with a custom flyout, the KeyboardAccelerators in the flyout aren't respected

### Describe the bug

I added this to my blank C# WinUI3 app:

```
protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
{
var m = new MenuFlyout();
m.Items.Add(new MenuFlyoutItem() { Text = "Dummy item 1" });
m.Items.Add(new MenuFlyoutItem() { Text = "Hello World" });

var ka = new KeyboardAccelerator()
{
Key = Windows.System.VirtualKey.H,
Modifiers = Windows.System.VirtualKeyModifiers.Control
};
ka.Invoked += KeyboardAccelerator_Invoked;
m.Items[1].KeyboardAccelerators.Add(ka);
Resources["TextControlCommandBarContextFlyout"] = m;
```

Expected: Pressing ctrl+h will cause KeyboardAccelerator_Invoked to get called.
Observed: It's not called.

I believe this works in System Xaml / UWP / CoreWindow, and that this problem is specific to islands and WinUI3 (which uses only islands).

### Why is this important?

This is not for an important scenario, I've filed it mostly for tracking.

### Steps to reproduce the bug

I added this to my blank C# WinUI3 app:

```
protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
{
var m = new MenuFlyout();
m.Items.Add(new MenuFlyoutItem() { Text = "Dummy item 1" });
m.Items.Add(new MenuFlyoutItem() { Text = "Hello World" });

var ka = new KeyboardAccelerator()
{
Key = Windows.System.VirtualKey.H,
Modifiers = Windows.System.VirtualKeyModifiers.Control
};
ka.Invoked += KeyboardAccelerator_Invoked;
m.Items[1].KeyboardAccelerators.Add(ka);
Resources["TextControlCommandBarContextFlyout"] = m;
```

### Actual behavior

Event is not raised and KeyboardAccelerator_Invoked is not called.

### Expected behavior

KeyboardAccelerator_Invoked should be called.

### Screenshots

_No response_

### NuGet package version

None

### Windows version

_No response_

### Additional context

_No response_

Start with the OnLaunched reproduction in the issue and trace how a custom TextControlCommandBarContextFlyout handles KeyboardAccelerators in a WinUI 3 desktop app. Compare the expected Ctrl+H behavior with the observed missing KeyboardAccelerator_Invoked call; done means the callback is raised for the custom flyout.

Written by the indexing model from the issue text.