AES cleanups and javadoc

core/java/src/net/i2p/crypto/AESEngine.java

core/java/src/net/i2p/crypto/CryptixAESEngine.java

core/java/src/net/i2p/crypto/CryptixAESKeyCache.java

core/java/src/net/i2p/crypto/ElGamalAESEngine.java