Retire encrypted flag in backups

Emmm, we retired `encrypted` flag and have a new field to handle encrypted secret. This makes sense. I think we can do a migration when import backups, and remove all logic about `encrypted` flag.

_Originally posted by @Sneezry in https://github.com/Authenticator-Extension/Authenticator/issues/485#issuecomment-632159860_

Start by tracing the backup import entry point and all uses of the retired `encrypted` flag. Confirm how the new encrypted-secret field is represented, then verify that imports migrate older backups and that no obsolete flag logic remains.

Written by the indexing model from the issue text.