Changed DnsPinger to use the system default DNS if linkprops doesn't
have a dns. This mirrors the behavior of the system overall.
Minor changes to wifiWatchdogService settings.
Change-Id: I8de73cf5bd24bc69343c7d9dc999d198195ec0ec
Complete rewrite of WifiWatchdogService.java. Checking for connectivity and managing wifi upon failure detection.
Change-Id: Ifcb8b5d7e0112cbc2f2282d76fdc93ea15527a44