ringaccount: remember if archive has a password

When saving the account archive, remember if it has a
password to avoid asking for a password if there is none.
If the information if unknown, assume there is a password
(the user can still enter en empty password in this case).

This information if exposed with the read-only account property
ConfProperties::ARCHIVE_HAS_PASSWORD

Change-Id: Ib19d6225b07b4e222dfd099f6922a874168af542
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>