Edit: found my saves, but how do I use them?
I just realize I didn't have a SD card but it still helped me.
Then you have to use the "My Files" app (This assumes a Samsung device), tap the SD card, select the files, copy them and go to the Powder Toy data folder (Internal storage/Android/*TPT folder*/data), put them in the save folder and boom. If not, look in the internal storage for something like "The-Powder-Toy" or something like that, insert the save and that should be it.
EDIT: Please delete this post.
This website has some tips: https://www.codenameone.com/blog/associating-your-app-with-file-extension-mime-types-iphone-android-windows.html#_android_specific_configuration