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/gc/space/space_test.h b/runtime/gc/space/space_test.h
index 17d7c87..7778871 100644
--- a/runtime/gc/space/space_test.h
+++ b/runtime/gc/space/space_test.h
@@ -62,7 +62,7 @@
       byte_array_class_ = self->GetJniEnv()->NewLocalRef(byte_array_class);
       EXPECT_TRUE(byte_array_class_ != nullptr);
     }
-    return reinterpret_cast<mirror::Class*>(self->DecodeJObject(byte_array_class_));
+    return self->DecodeJObject(byte_array_class_)->AsClass();
   }
 
   mirror::Object* Alloc(space::MallocSpace* alloc_space,