In addition to Mark's answer, I googled "android text editor with dropbox sync" and found & installed Plain.txt. In your dropbox, it creates an Apps folder with a Plain.Txt subfolder. That does seem to sync seamlessly & instantly with all other devices, but it's an extremely minimal editor. It doesn't have a save button. You have to quit the app & come back in to save and see changes. Changes made on some other device meanwhile will get you a conflicting copy. All in all, not a workable solution. Pity. Off you go, Plain.txt.
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