PositionInSet and SizeOfSet do not take into account hidden/collapsed elemnts

### Description

Elements that present a collection of items (such as a Menu with MenuItems or a ListView with ListViewItems) do not take into account non-visible elements when calculating PositionInSet and SizeOfSet. This results in screen readers announcing incorrect information.

### Reproduction Steps

Create a WPF project that contains one such element. For example, consider the following XAML:

```xaml

```

Compile this project, open Windows Narrator, and select one of the two visible items in the list.

### Expected behavior

When selecting the first item, Windows Narrator should announce "1 of 2." When selecting the second item (C), Windows Narrator should announce "2 of 2."

### Actual behavior

When selecting the first item, Windows Narrator announces "1 of 3." When selecting the second item (C), Windows Narrator announces "3 of 3."

### Regression?

_No response_

### Known Workarounds

_No response_

### Impact

This is an accessibility issue, since screen readers announce incorrect information.

### Configuration

_No response_

### Other information

_No response_