The directory we were previously saving backups to on the external SD
card is actually deleted upon app uninstall and/or clearing the app's
data. There's also no reliable way to write to the root of an external
SD card (that isn't comically inconvenient), so for now it's safer if we
just move back to getting the regular 'ol standard external storage
directory (which is likely internal storage, despite its name).
Fixes#7845
