[xabt] suppress XA0101 for Razor class libraries

[jonathanpeppers]

Context: https://github.com/dotnet/sdk/blob/1ed2f8e9751753a1d249d129884a669363c1b832/src/StaticWebAssetsSdk/Sdk/Sdk.StaticWebAssets.StaticAssets.ProjectSystem.props#L30-L31
Fixes: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2533207

1. Create a dotnet new maui-blazor-web project.

2. Open the *.Shared.csproj, the Razor class library project.

3. Change the $(TargetFramework) from net9.0 to net9.0-android.

4. Build the project.

Get the warnings:

hellomauiblazor.Shared net9.0-android succeeded with 11 warning(s) (0.4s) → hellomauiblazor.Shared\bin\Debug\net9.0-android\hellomauiblazor.Shared.dll
wwwroot\app.css : warning XA0101: @(Content) build action is not supported
wwwroot\bootstrap\bootstrap.min.css : warning XA0101: @(Content) build action is not supported
wwwroot\bootstrap\bootstrap.min.css.map : warning XA0101: @(Content) build action is not supported
wwwroot\favicon.png : warning XA0101: @(Content) build action is not supported
Layout\MainLayout.razor : warning XA0101: @(Content) build action is not supported
Layout\NavMenu.razor : warning XA0101: @(Content) build action is not supported
Pages\Counter.razor : warning XA0101: @(Content) build action is not supported
Pages\Home.razor : warning XA0101: @(Content) build action is not supported
Pages\Weather.razor : warning XA0101: @(Content) build action is not supported
Routes.razor : warning XA0101: @(Content) build action is not supported
_Imports.razor : warning XA0101: @(Content) build action is not supported

Reviewing the code in the .NET SDK for Razor projects, they include most of these files such as:

<!-- Publish everything under wwwroot, all JSON files, all config files and all Razor files -->
<Content Include="wwwroot\**" ExcludeFromSingleFile="true" CopyToPublishDirectory="PreserveNewest" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />

%(ExcludeFromSingleFile) looks like a useful metadata here, as Android is basically "single file mode" no matter what. We set $(EnableSingleFileAnalyzers)=true by default, so we consider mobile a "single file" platform.

Let's suppress the XA0101 warning for files that have %(ExcludeFromSingleFile) metadata, as this seems actually appropriate and solves the warnings in Razor class libraries.