[th/wifi-scan-for-seen-bssids] wifi: fix eager background Wi-Fi scanning with seen-bssids
"wifi.seen-bssids" looks like a regular property, but it is not. Unlike almost all other properties, it does not contain user configuration, rather it gets filled by the daemon.
The values are stored in "/var/lib/NetworkManager/seen-bssids" file, and
in the daemon they are maintained separately from the profile. Only
before exporting the profile on D-Bus, the value gets merged (see
NM_SETTINGS_CONNECTION_GET_PRIVATE(self)->>getsettings_cached and
nm_connection_to_dbus_full().
Hence, looking at nm_setting_wireless_get_num_seen_bssids() is not
working. Fix that.
Fixes: 0f320333 ('wifi: roam aggressively if we on a multi-AP network')