Merge: Symetric [] & []= in android::shared_preferences

Now SharedPreferences[]= always serialize, and [] deserialize. This would be the default dehavior for a Nit application. The other getter/setter for primitive types are still available but must be used directly.

Pull-Request: #577
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Frédéric Vachon <fredvac@gmail.com>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>