gis-goa-page: Port to new async provider API

The API changed with https://git.gnome.org/browse/gnome-online-accounts/commit/?id=1a8bfdf90fd24e9bbaeeae15ff9f16e847a6b48e

Update this page to be async.  Ideally we'd have a loading API, but
eh.

https://bugzilla.gnome.org/show_bug.cgi?id=706608