LocaleTracker: Add null check before accessing WifiManager

 * WiFi service is not considered as a core service so it won't start in
   min framework, but LocaleTracker accesses to WiFi service's API.
   This would lead to a fatal exception and trigger RescueParty.
 * Add null check before accessing WifiManager to avoid the crash.

Change-Id: Ib1e9aef5a34cb8cfc33f7aa6f242807bc6ef2c46