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/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index 56839f8..5630b08 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -898,8 +898,10 @@
           *dex_file,
           Runtime::Current()->GetLinearAlloc())));
       Handle<mirror::Class> klass(hs2.NewHandle(
-          class_linker->ResolveType(*dex_file, exception_type_idx, dex_cache,
-                                    NullHandle<mirror::ClassLoader>())));
+          class_linker->ResolveType(*dex_file,
+                                    exception_type_idx,
+                                    dex_cache,
+                                    ScopedNullHandle<mirror::ClassLoader>())));
       if (klass.Get() == nullptr) {
         const DexFile::TypeId& type_id = dex_file->GetTypeId(exception_type_idx);
         const char* descriptor = dex_file->GetTypeDescriptor(type_id);