The main goal is to avoid screen flickering at app start time, when the preference references are downloaded and notify a refresh (again and again everyday, with the same language).
New file:
* `downloadable_string.dart`: Downloadable String that may be stored (and compared to the previous value).
Impacted files:
* `main.dart`: refactored
* `product_preferences.dart`: created explicit methods `init` and `refresh`; now stores locally the downloaded strings, that will be reused for the next app start
* `user_preferences_page_test.dart`: refactored
