[foldable] `SpanMode` not updated when app spanned/unspanned

### Description

SpanMode property should update each time the app is spanned or unspanned.

An event isn't firing (or something...) and the mode property does not get updated with the current mode

### Steps to Reproduce

1. Repros with the Sample App in the .NET MAUI source repo
2. Other Concepts -> Foldable -> shows a view with a TwoPaneView
3. The `SpanMode` property never changes from **SinglePane**

### Version with bug

Unknown/Other (please specify)

### Last version that worked well

Unknown/Other

### Affected platforms

Android

### Affected platform versions

Android 11

### Did you find any workaround?

No workaround apparent

### Relevant log output

_No response_

Start with the .NET MAUI source repository's Sample App at Other Concepts → Foldable, where the TwoPaneView demonstrates the issue. Reproduce spanning and unspanning on Android 11, then trace why SpanMode remains SinglePane; done means the property reflects the current mode after each transition.

Written by the indexing model from the issue text.