Xamarin.Android.Tools.Aidl.CSharpCodeGenerator needs unit tests.

[jonpryor]

Xamarin.Android.Tools.Aidl.CSharpCodeGenerator needs (more?) "proper" unit tests.

For inspiration, see:

• jonpryor/java.interop@9e90253
• [generator] Add --with-javadoc-xml=FILE support. java-interop#687

This is so that we don't need to go through the MSBuild integration tests in https://github.com/xamarin/xamarin-android/blob/f27e7cd3d6b459940d2b9c494ae3afc1b6210ab8/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/AidlTest.cs.