Disable storing unencrypted backups in the persistence storage when encryption is enabled

Now that `toybox` is out of the picture and the archives are created in Java (and therefore, we've more control over it), it's no longer necessary to store the unencrypted backups in the persistence storage before encrypting them. In fact, we've already added support for encrypting streams in our `Crypto` framework. So, the only obstacle is rewriting the back up/restore procedures to support this.