Activites are not stateless. They can simply be torn down and restored at any time. You should be using bundles to store and restore your activity state. You don't want to persist the view state to disk anyway.
you can't put many bytes in bundles or else you run into problems. The correct approach is to just store the id or the minimum needed in order to restore the view, most of the info will either come from the server, a cache or a database..