Fix dotnet/android build

Context: dotnet/android#9750

dotnet/android#9750 attempts to test #1302, and it
failed to build!

    …/src/Mono.Android/obj/Release/net10.0/android-35/mcw/Java.Util.Jar.Pack200.cs(229,54): error CS1109:
    Extension methods must be defined in a top level static class; IPackerExtensions is a nested class

Indeed, what we actually had was:

    abstract partial class Pack200 {
      public partial interface IPacker { /* … */ }

      static partial class IPackerExtensions {
        public static void Pack (this IPacker packer, Stream output, Stream input)
        {
          …
        }
      }
    }

`static` classes containing extension methods must be top-level,
and `IPackerExtensions` is instead nested within `Pack200`.

Fix `generator` so that this doesn't happen.

Author: jonpryor

tools/generator/SourceWriters/BoundClass.cs

@@ -83,7 +83,7 @@ public BoundClass (ClassGen klass, CodeGenerationOptions opt, CodeGeneratorConte
 				NestedTypes.Add (ic);
 
 			// Sibling classes
-			if (!klass.AssemblyQualifiedName.Contains ('/')) {
+			if (!klass.AssemblyQualifiedName.Contains ('+')) {
 				foreach (InterfaceExtensionInfo nestedIface in klass.GetNestedInterfaceTypes ())
 					if (nestedIface.Type.Methods.Any (m => m.CanHaveStringOverload) || nestedIface.Type.Methods.Any (m => m.Asyncify))
 						sibling_types.Add (new InterfaceExtensionsClass (nestedIface.Type, nestedIface.DeclaringType, opt));

tools/generator/SourceWriters/BoundInterface.cs

@@ -78,7 +78,7 @@ public BoundInterface (InterfaceGen iface, CodeGenerationOptions opt, CodeGenera
 			if (iface.IsConstSugar (opt))
 				return;
 
-			if (!iface.AssemblyQualifiedName.Contains ('/')) {
+			if (!iface.AssemblyQualifiedName.Contains ('+')) {
 				if (iface.Methods.Any (m => m.CanHaveStringOverload) || iface.Methods.Any (m => m.Asyncify))
 					post_sibling_types.Add (new InterfaceExtensionsClass (iface, null, opt));
 			}