[foldable] `SpanModeStateTrigger` not responding to span/unspan events

### Description

Implementing a state trigger with `TwoPaneView` doesn't seem to work. Could be related to #5852 since it's probably an event not getting fired somewhere; might be fixed by same code too 🤞

### 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. Placing the XAML below on the `Pane2` `StackLayout`

```xaml

```

### Link to public reproduction project repository

https://github.com/dotnet/maui

### Version with bug

7.0 Release Candidate 2

### Last version that worked well

Unknown/Other

### Affected platforms

Android

### Affected platform versions

Android 12L

### Did you find any workaround?

_No response_

### Relevant log output

_No response_

Run the .NET MAUI Sample App and open Other Concepts → Foldable to reproduce the TwoPaneView behavior with the shown SpanModeStateTrigger XAML. Trace how span/unspan events reach SpanModeStateTrigger and compare the suspected event path with issue #5852. Done means the SinglePane, Wide, and Tall visual states update when the foldable changes mode on Android 12L.

Written by the indexing model from the issue text.