1) Move the attachment structures into the encrypted message body.
2) Encrypt attachments with symmetric keys transmitted in the
encryptd attachment pointer structure.
3) Correctly handle asynchronous decryption and categorization of
encrypted push messages.
TODO: Correct notification process and network/interruption
retries.
1) Add encryption support for the transport layer. This obscures
metadata from the push messaging provider.
2) Better support the direction multiple destination messages is
headed (one unique message per recipient).
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.
