OnBackPressed not called with modal open

### Description

After upgrading from .NET8 to .NET9 the android activity's `onBackPressed` is no longer called if a modal page is open.

### Steps to Reproduce

1. Open the repo project on master branch (.NET8)
2. Click the button to open a modal page
3. Press the android back button
4. Observe `MainActivity.OnBackPressed` is called first then `MainPage.OnBackButtonPressed` is called.
5. Switch to net9 branch (.NET9)
6. Redo the same steps as before.
7. Observe `MainPage.OnBackButtonPressed` is called but not `MainActivity.OnBackPressed`

### Link to public reproduction project repository

https://github.com/WillMDK/modalnavigation

### Version with bug

9.0.40 SR4

### Is this a regression from previous behavior?

Yes, this used to work in .NET MAUI

### Last version that worked well

8.0.100 SR10

### Affected platforms

Android

### Affected platform versions

Android 14

### Did you find any workaround?

_No response_

### Relevant log output

```shell
.NET8:
[0:] MainActivity OnBackPressed called
[0:] MainPage OnBackButtonPressed called

.NET9:
[0:] MainPage OnBackButtonPressed called
```

Start with the public reproduction project at github.com/WillMDK/modalnavigation and compare its master (.NET 8) and net9 branches on Android 14. Trace the back-button flow through MainActivity.OnBackPressed and MainPage.OnBackButtonPressed; done means the .NET 9 behavior restores the expected callback sequence and the reproduction logs match .NET 8.

Written by the indexing model from the issue text.