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_test.cc b/runtime/class_linker_test.cc
index 387ac0a..59a43ee 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -855,7 +855,7 @@
   // Validate that the "value" field is always the 0th field in each of java.lang's box classes.
   // This lets UnboxPrimitive avoid searching for the field by name at runtime.
   ScopedObjectAccess soa(Thread::Current());
-  NullHandle<mirror::ClassLoader> class_loader;
+  ScopedNullHandle<mirror::ClassLoader> class_loader;
   mirror::Class* c;
   c = class_linker_->FindClass(soa.Self(), "Ljava/lang/Boolean;", class_loader);
   EXPECT_STREQ("value", c->GetIFieldsPtr()->At(0).GetName());
@@ -1101,7 +1101,7 @@
 
 TEST_F(ClassLinkerTest, ValidatePredefinedClassSizes) {
   ScopedObjectAccess soa(Thread::Current());
-  NullHandle<mirror::ClassLoader> class_loader;
+  ScopedNullHandle<mirror::ClassLoader> class_loader;
   mirror::Class* c;
 
   c = class_linker_->FindClass(soa.Self(), "Ljava/lang/Class;", class_loader);