If someone has set Signal as the default SMS but has cleared data or
otherwise reset the app's storage state, it can get into a weird
situation. Notably, it'll crash because SmsReceiveJob.onRun() expects
Recipient.self() to be available.
However, it also makes it impossible to get the registration SMS,
because the app won't post a notification for the code.
This change will post notifications and SmsRetriever broadcasts for
relevant SMS messages.
