This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
If you don't want a generated key, then don't use the KeyGenerator class :-)
Construct a SecretKeySpec instead, and pass that to the Cipher.init method. If the byte is of length 16 you'll get AES-128, if its's of length 32 then AES-256 will be used (assuming your JCE provider supports that).