[nkohari]

Have you considered adding logins? :)

But seriously, this is literally the problem that logins were designed to solve. With a login, you are defining an external concept of "user" that is distinct from the device.

Available security factors are: something you are, something you possess, something you know. Right now, you're using something you possess (the device). The problem is, if I lose the device, I've lost my security credential, and no longer "own" my account.

To fix this, you could collect an email, and when installing your app, you could prompt the user if they want to use an existing account or create another. But then, you probably want to add some security in the form of a password... and then you've created a login.