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/class_linker-inl.h b/runtime/class_linker-inl.h
index a5d10b2..ea1afa8 100644
--- a/runtime/class_linker-inl.h
+++ b/runtime/class_linker-inl.h
@@ -30,7 +30,7 @@
 namespace art {
 
 inline mirror::Class* ClassLinker::FindSystemClass(Thread* self, const char* descriptor) {
-  return FindClass(self, descriptor, NullHandle<mirror::ClassLoader>());
+  return FindClass(self, descriptor, ScopedNullHandle<mirror::ClassLoader>());
 }
 
 inline mirror::Class* ClassLinker::FindArrayClass(Thread* self, mirror::Class** element_class) {