Fix copying null reference across themes

When themes have two different AssetManagers, we only copy attributes
that exist in both the themes. If the value of the attribute or
reference is equal to 0 (a null reference but not using the TYPE_NULL
type), do not attempt to fix the package id of that null reference.

Bug: 119522708
Test: manual test of broken app
Change-Id: Id615d5a47b7f121b08bbba885d106b9cd3f54afc