ObjPtr<>-ify mirror::Object::As*(), fix stale refs in test.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: Ia04c4996f5cbfa1f26acbadcd747342c57b1aac3
diff --git a/runtime/native/java_lang_System.cc b/runtime/native/java_lang_System.cc
index e4bc8ce..63cbd2c 100644
--- a/runtime/native/java_lang_System.cc
+++ b/runtime/native/java_lang_System.cc
@@ -127,9 +127,9 @@
             dstPos, ObjPtr<mirror::LongArray>::DownCast(srcArray), srcPos, count);
         return;
       case Primitive::kPrimNot: {
-        mirror::ObjectArray<mirror::Object>* dstObjArray =
+        ObjPtr<mirror::ObjectArray<mirror::Object>> dstObjArray =
             dstArray->AsObjectArray<mirror::Object>();
-        mirror::ObjectArray<mirror::Object>* srcObjArray =
+        ObjPtr<mirror::ObjectArray<mirror::Object>> srcObjArray =
             srcArray->AsObjectArray<mirror::Object>();
         dstObjArray->AssignableMemmove(dstPos, srcObjArray, srcPos, count);
         return;