portalHelper: Reduce the connectivity check rate limit

portalHelper: Reduce the connectivity check rate limit

30 seconds is much too long for GNOME Shell to notice that full
connectivity has been achieved. The portal helper is expected to be
closed once connectivity is achieved.

Note this does NOT mean we unconditionally spam the connectivity check
server every 5 seconds. A connectivity check is still only scheduled
when an actual navigation occurs.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3813>
(cherry picked from commit 1c80a4f213faef5170c3af6720d1e6d623fc45ad)