Move mirror::Class to use ObjPtr

Leave the return types as non ObjPtr for now. Fixed moving GC bugs
in tests.

Test: test-art-host

Bug: 31113334

Change-Id: I5da1b5ac55dfbc5cc97a64be2c870ba9f512d9b0
diff --git a/runtime/jni_internal.cc b/runtime/jni_internal.cc
index 273c67d..d00e3a3 100644
--- a/runtime/jni_internal.cc
+++ b/runtime/jni_internal.cc
@@ -629,7 +629,7 @@
           WellKnownClasses::StringInitToStringFactory(soa.DecodeMethod(mid)));
       return CallStaticObjectMethodV(env, WellKnownClasses::java_lang_StringFactory, sf_mid, args);
     }
-    mirror::Object* result = c->AllocObject(soa.Self());
+    ObjPtr<mirror::Object> result = c->AllocObject(soa.Self());
     if (result == nullptr) {
       return nullptr;
     }
@@ -656,7 +656,7 @@
           WellKnownClasses::StringInitToStringFactory(soa.DecodeMethod(mid)));
       return CallStaticObjectMethodA(env, WellKnownClasses::java_lang_StringFactory, sf_mid, args);
     }
-    mirror::Object* result = c->AllocObject(soa.Self());
+    ObjPtr<mirror::Object> result = c->AllocObject(soa.Self());
     if (result == nullptr) {
       return nullptr;
     }