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/mirror/field-inl.h b/runtime/mirror/field-inl.h
index 445f23f..940e824 100644
--- a/runtime/mirror/field-inl.h
+++ b/runtime/mirror/field-inl.h
@@ -48,7 +48,7 @@
       self->ClearException();
     }
   }
-  auto ret = hs.NewHandle(static_cast<Field*>(StaticClass()->AllocObject(self)));
+  auto ret = hs.NewHandle(static_cast<Field*>(StaticClass()->AllocObject(self).Ptr()));
   if (UNLIKELY(ret.Get() == nullptr)) {
     self->AssertPendingOOMException();
     return nullptr;