Move mirror::ObjectArray to ObjPtr

Also reduce calls to ObjPtr::Ptr.

Bug: 31113334

Test: test-art-host -j32

Change-Id: I73e1b356972bb079b66332f00f7f07451601044e
diff --git a/runtime/reflection.cc b/runtime/reflection.cc
index 098eb03..72db827 100644
--- a/runtime/reflection.cc
+++ b/runtime/reflection.cc
@@ -772,7 +772,7 @@
       }
       return false;
     }
-    unboxed_value->SetL(o.Ptr());
+    unboxed_value->SetL(o);
     return true;
   }
   if (UNLIKELY(dst_class->GetPrimitiveType() == Primitive::kPrimVoid)) {