Paper key is saved even if I don't click "Done"

I suspected that checking "I've written this down" and pressing "Done" would be needed to save the paper key. Otherwise, it also adds the key if e.g. you've landed on that screen by mistake.

Steps to reproduce:
1. Go to Devices tab.
2. Press "Add new...", select "New paper key"
3. Press "Back".

What I expected: Paper key generation is cancelled, no new key is added.
What happened: New paper key appeared. It's not only a visual bug, they are visible on keybase.io, and I got an e-mail.

I've recorded a screencap here: https://gfycat.com/FrigidImmenseAfricangroundhornbill

Start by tracing the Devices tab flow for creating a new paper key and compare the Back path with the Done path. Reproduce the issue using the listed steps, then verify that backing out cancels key generation and does not create a visible key or send an email.

Written by the indexing model from the issue text.