[NET 11] AppBar is not highlighted when NavigationBarColor is customized

### Description

In Material 3 , when a scroll action happens, the AppBar area will be highlighted. But if customize the toolbar color in both Navigation and shell pages. The highlight effect is gone.

### Steps to Reproduce

1. Run the attached repro.
2. Scroll the content and notice the hightlight.
3. uncomment the BarBackgroundColor in App.xaml.cs.
4. deploy again and scroll. Notice no highlight is happening.

### Link to public reproduction project repository

[MauiAppNet11.zip](https://github.com/user-attachments/files/31470333/MauiAppNet11.zip)

### Version with bug

11.0.0-preview.7.26426.3

### Is this a regression from previous behavior?

No, this is something new

### Last version that worked well

Unknown/Other

### Affected platforms

Android

### Affected platform versions

_No response_

### Did you find any workaround?

_No response_

### Relevant log output
| Default | Customized |
|--|--|
| | |