This commit adds the contact name and the readable date (and time) to
the plaintext export of every message. That's because that is how SMS
Backup and Restore does it, so this commit makes Signal more compatible.
Closes#6452
// FREEBIE
1) Move all the crypto classes from securesms.crypto.
2) Move all the crypto storage from securesms.database.keys
3) Replace the old imported BC code with spongycastle.
1) Allow imports from the stock SMS database at any time.
2) Provide plaintext export support, in a format compatible with
the "SMS Backup And Restore" app.
3) Fix the DB weirdness on encrypted restore that previously
required killing the app.
