Rename NullHandle to ScopedNullHandle

This makes it clearer that is invalid to do things like:
Handle<T> h = ScopedNullHandle<T>();

Bug: 26233305
Change-Id: I6d8f54eae01ec2e901cb7043afa853ea77db79fe
diff --git a/runtime/native/java_lang_Class.cc b/runtime/native/java_lang_Class.cc
index 14d284e..1977481 100644
--- a/runtime/native/java_lang_Class.cc
+++ b/runtime/native/java_lang_Class.cc
@@ -653,7 +653,8 @@
     }
   }
   auto* constructor = klass->GetDeclaredConstructor(
-      soa.Self(), NullHandle<mirror::ObjectArray<mirror::Class>>());
+      soa.Self(),
+      ScopedNullHandle<mirror::ObjectArray<mirror::Class>>());
   if (UNLIKELY(constructor == nullptr)) {
     soa.Self()->ThrowNewExceptionF("Ljava/lang/InstantiationException;",
                                    "%s has no zero argument constructor",