wlan: Provide runtime option to enable or disable rssi threshold (settings)

RSSI threshold check in AutoJoin feature will not allow
to connect if AP's RSSI is low, this option will allow user
to enable/disable RSSI threshold check through advance settings.

Conflicts:
        res/xml/wifi_advanced_settings.xml
        src/com/android/settings/wifi/AdvancedWifiSettings.java

Change-Id: I82d7082a97cfc2dc75e0b14221b176697060e05d
CRs-Fixed: 710284