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.
