imap-smtp: Use goa_utils_get_credentials in the provider
imap-smtp: Use goa_utils_get_credentials in the provider

Less straightforward that the other ones as the username is not
obtained through goa_account_get_identity but comes from the config
file. Also, the credentials GVariant was used twice, once for IMAP and
once for SMTP. I chose to call goa_utils_get_credentials twice instead.

https://bugzilla.gnome.org/show_bug.cgi?id=755316

ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts