route files named encrypted.asc to be decrypted, K-9 turns PGP/MIME into it

K-9 turns PGP/MIME messages into an encrypted.asc attachment, so route this
to be decryptable and viewable.
for now, route all application/pgp-encrypted data thru FileHandlerActivity

Once the encrypt Activity gets fixed up, this IntentFilter should be in
that Activity.