SelectedIndex is set to internalIndex when parent element becomes visible

### Describe the bug

When a parent element of a segmented control is invisible at the loading of a view, the databinding is not triggered until it becomes visible. This updates the value of the SelectedIndex and sets it to 0.

In the following setup:
```xaml

````
The view is loaded:
- Enabled == false so the visibility of the stackpanel is hidden
- ItemsSource is populated (OnPropertyChanged is fired)
- Property bound to SelectedIndex is set to 1 (in this examples case that is "NotNullVM.UnitTypeFilterButtonSelection") (OnPropertyChanged is fired)
- Enabled = true so the stackpanel becomes visible
- Then the segmented control detects the ItemsSource is updated, it reacts by setting the SelectedIndex to "_internalSelectedIndex" (I am not 100% of the property name at this point) which is 0 at this point
- This updates the bound SelectedIndex property (in this examples case that is "NotNullVM.UnitTypeFilterButtonSelection") and sets it to 0.

We have verified, if we start with Enabled==true (so the parent stackpanel is visible), it al works properly.

### Steps to reproduce

```text
The view is loaded:
1. Enabled == false so the visibility of the stackpanel is hidden
2. ItemsSource is populated (OnPropertyChanged is fired)
3. Property bound to SelectedIndex is set to 1 (in this examples case that is "NotNullVM.UnitTypeFilterButtonSelection") (OnPropertyChanged is fired)
4. Enabled = true so the stackpanel becomes visible
5. Then the segmented control detects the ItemsSource is updated, it reacts by setting the SelectedIndex to "_internalSelectedIndex" (I am not 100% of the property name at this point) which is 0 at this point
6. This updates the bound SelectedIndex property (in this examples case that is "NotNullVM.UnitTypeFilterButtonSelection") and sets it to 0.
```

### Expected behavior

I expect the SelectedIndex to only update if it falls outside the range of the newly available options.

### Screenshots

It appears to come from here according to the stacktrace (this is the Segmented.cs file):

### Code Platform

- [ ] UWP
- [X] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [X] Windows 10 2004 (Build 19041)
- [X] Windows 10 20H2 (Build 19042)
- [X] Windows 10 21H1 (Build 19043)
- [X] Windows 10 21H2 (Build 19044)
- [X] Windows 10 22H2 (Build 19045)
- [X] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [X] Windows 10, version 2004 (Build 19041)
- [ ] Windows 10, version 2104 (Build 20348)
- [ ] Windows 11, version 22H2 (Build 22000)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

2022

### Visual Studio Build Number

17.9.2

### Device form factor

Desktop

### Additional context

We are using MvvmCross framework.
As we are using Observables, and this data is loaded as the result of a async get request, it is loaded after ViewAppread() has already triggered. (the observable OnNext is executed on the MainThread thought so that shouldn't be a problem).

### Help us help you

Yes, but only if others can assist.

Inizia leggendo Segmented.cs nella posizione identificata dallo stack trace e riproduci la sequenza con il parent nascosto, ItemsSource popolato e SelectedIndex impostato. Aggiungi una copertura di regressione per preservare un SelectedIndex valido quando il controllo diventa visibile; il lavoro è completo quando il valore cambia solo se è al di fuori delle opzioni disponibili.

Scritto dal modello di indicizzazione a partire dal testo della issue.