Properly parse and set PreferredAEADCipherSuites subpacket

[vanitasvitae]

In #1410 I apparently forgot to change some places away from PreferredAlgorithms to PreferredAEADCiphersuites.

This PR fixes these cases.

[dghgit]

Would you deprecate the current methods and add setPreferredAEADCiphersuites and getPreferredAEADCiphersuites instead? Thanks to the latest GPG role out a lot of people seem to be using AEAD now - I think it's too late to break the API on this one, there's going to be a lot of third party code depending on the current methods.

[vanitasvitae]

Sure, I can change that :)
It is kind of fitting, that the old preferences packet type (here called Preferred Encryption Modes) was deprecated in GnuPGs branch of the specification as well: https://www.ietf.org/archive/id/draft-koch-openpgp-2015-rfc4880bis-02.html#name-preferred-encryption-modes

Edit: I reintroduced the old methods and added a disclaimer about their deprecation.
I also re-added the GnuPG-style PREFERRED_ENCRYPTION_MODES algorithm preferences subpacket type.

[vanitasvitae]

Let me add some more tests for this, so that we can be sure that both methods are not interfering with another.