[class-parse] fix class visibility XML output.

As part of fixing MSBuild test regression[*1], I noticed that class-parse
does not generate correct visibility for class/interface elements, which
could affect further API metadata processing. It should not generate
"private" for nested types when it should be "" (package private).

[*1] it should be ported to OSS...

Author: atsushieno

src/Xamarin.Android.Tools.Bytecode/XmlClassDeclarationBuilder.cs

@@ -39,7 +39,7 @@ public XElement ToXElement ()
 					new XAttribute ("name",                     GetThisClassName ()),
 					new XAttribute ("jni-signature",            "L" + classFile.ThisClass.Name.Value + ";"),
 					new XAttribute ("static",                   classFile.IsStatic),
-					new XAttribute ("visibility",               GetVisibility (classFile.Visibility)),
+					new XAttribute ("visibility",               GetClassVisibility (classFile.Visibility)),
 					GetTypeParmeters (signature == null ? null : signature.TypeParameters),
 					GetImplementedInterfaces (),
 					GetConstructors (),