Allow upgrade from Bouncy Castle 149 to 150

This fixes our dependency on BC 149's outdated OCB implementation. I copied the OCBBlockCipher class from BC149 so that we can can still support the old nonce length as long as we need to. This also makes it trivially easy to update to the new nonce length for OCB. 
Newlines should not be modified if possible - it introduces needless changeset noise. Could you restore the trailing newline here please?