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/compiler/oat_test.cc b/compiler/oat_test.cc
index 451aa68..7b7d46c 100644
--- a/compiler/oat_test.cc
+++ b/compiler/oat_test.cc
@@ -224,8 +224,9 @@
     }
 
     const char* descriptor = dex_file.GetClassDescriptor(class_def);
-    mirror::Class* klass = class_linker->FindClass(soa.Self(), descriptor,
-                                                   NullHandle<mirror::ClassLoader>());
+    mirror::Class* klass = class_linker->FindClass(soa.Self(),
+                                                   descriptor,
+                                                   ScopedNullHandle<mirror::ClassLoader>());
 
     const OatFile::OatClass oat_class = oat_dex_file->GetOatClass(i);
     CHECK_EQ(mirror::Class::Status::kStatusNotReady, oat_class.GetStatus()) << descriptor;