Implement lightweight method to generate AES keys.

[Vladimir Dzhuvinov]

This applies to the AES crypto utility class. The current implementation relies on loading a full-blown BouncyCastle JCE provider for that. A more efficient approach would be to use a SecureRandom generator directly. The thread-safety of that approach should be ensured if the generator is to be reused (for better performance).

[Vladimir Dzhuvinov]

Opposite approach is chosen, switch to JCA interface.