Warning, you will use your boosters if you uninstall your game or change devices

That's such a shame 🥺. The only way boosters could be saved is if they would be linked to the King/FB account and not on the internal storage device as some have already said. However, if the same would apply to gold bars and boosters, then you wouldn't be able to use boosters when you're offline (since some people actually play that way as well because they have a limited data consume).