wifi: roam more aggressively

-80dBm is a very weak signal and we should roam sooner.
iwd[1] and iOS[2] default to -70dBm, Android[3] default to -73dBm.
So lets change the roaming threshold to -70dBm.

[1] https://git.kernel.org/pub/scm/network/wireless/iwd.git/commit/?id=45a51613c4c24067fe0e35b4c084a50124282b36
[2] https://support.apple.com/en-us/HT203068
[3] 68a4599a37/core/res/res/values/config.xml (609)
    dacacbf69d/service/java/com/android/server/wifi/ScoringParams.java (218)
    dacacbf69d/service/java/com/android/server/wifi/ScoringParams.java (325)
    ee0920fb68/service/java/com/android/server/wifi/WifiNetworkSelector.java (149)

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/201
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/merge_requests/278

src/supplicant/nm-supplicant-config.c

@@ -585,7 +585,7 @@ nm_supplicant_config_add_bgscan (NMSupplicantConfig *self,
 	 * (b) since EAP/802.1x isn't used and thus there are fewer steps to fail
 	 * during a roam, we can wait longer before scanning for roam candidates.
 	 */
-	bgscan = "simple:30:-80:86400";
+	bgscan = "simple:30:-70:86400";
 
 	/* If using WPA Enterprise or Dynamic WEP use a shorter bgscan interval on
 	 * the assumption that this is a multi-AP ESS in which we want more reliable