Currently if we detect that a scan finished in
_scan_notify_is_scanning(), we call immediately _scan_kickoff() (which
might start a new scan) and then we check again whether the device can
autoactivate or whether to remove the wifi-scan pending action.

This means that if the scan takes long enough, when
_scan_notify_is_scanning() is called, it is already time to start
another scan and the device activation will be delayed. It will be
delayed until the scan duration becomes shorter than the
exponentially-growing periodic scan interval.

Fix this by delaying the next scan immediately after a scan result.

Co-authored-by: Thomas Haller <thaller@redhat.com>

#574
(cherry picked from commit 16d649ea)
(cherry picked from commit 94044c74)