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_writer.cc b/compiler/oat_writer.cc
index 2b2f0e8..53ac77b 100644
--- a/compiler/oat_writer.cc
+++ b/compiler/oat_writer.cc
@@ -737,7 +737,7 @@
         *dex_file_,
         it.GetMemberIndex(),
         dex_cache,
-        NullHandle<mirror::ClassLoader>(),
+        ScopedNullHandle<mirror::ClassLoader>(),
         nullptr,
         invoke_type);
     if (method == nullptr) {