Generated partial App class in XamlTypeInfo.g.cs has fixed "public" access modifier

### Describe the bug

When a WinUI 3 app has the [CA1515: Consider making public types internal](https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1515) code rule enabled, the `App` class can not be made `internal` due to its partial class generated in XamlTypeInfo.g.cs is made `public`.

### Steps to reproduce the bug

1. Create a blank WinUI 3 app with VisualStudio's WinUI 3 app template.
2. Go to App.xaml.cs, change the `App` class's access modifier from public to internal.
3. Build the project.
4. Build fails with the following error:
```
Error (active) CS0262 Partial declarations of 'App' have conflicting accessibility modifiers
```

### Expected behavior

App should be able to successfully build when change App to internal.

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.6.1: 1.6.240923002

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

Upon search there are 3 generated partial classes for App. While the following two do not have access modifier:
`partial class App : global::Microsoft.UI.Xaml.Application`

.\obj\x64\Debug\net8.0-windows10.0.22621.0\win-x64\Views\App.g.cs
.\obj\x64\Debug\net8.0-windows10.0.22621.0\win-x64\Views\App.g.i.cs

the generated partial class in
.\obj\x64\Debug\net8.0-windows10.0.22621.0\win-x64\XamlTypeInfo.g.cs
has `public partial class App : global::Microsoft.UI.Xaml.Markup.IXamlMetadataProvider`.

Reproduce the issue with a blank WinUI 3 app by changing App.xaml.cs from public to internal, then inspect the generated App declarations in App.g.cs, App.g.i.cs, and XamlTypeInfo.g.cs. Trace how XamlTypeInfo.g.cs assigns accessibility and verify the project builds successfully with App declared internal.

Written by the indexing model from the issue text.