Move remaining jobject related functions to use ObjPtr

Also added ObjPtr::DownCast.

Bug: 31113334

Test: test-art-host

Change-Id: I59c253211dc435579ffdfd49f856861ab13d262c
diff --git a/runtime/dex_file_annotations.cc b/runtime/dex_file_annotations.cc
index 367603e..f0d3909 100644
--- a/runtime/dex_file_annotations.cc
+++ b/runtime/dex_file_annotations.cc
@@ -611,7 +611,7 @@
   }
   Handle<mirror::Object> value_object(hs.NewHandle(annotation_value.value_.GetL()));
 
-  mirror::Class* annotation_member_class =
+  ObjPtr<mirror::Class> annotation_member_class =
       WellKnownClasses::ToClass(WellKnownClasses::libcore_reflect_AnnotationMember);
   Handle<mirror::Object> new_member(hs.NewHandle(annotation_member_class->AllocObject(self)));
   mirror::Method* method_obj_ptr;