Check dns for www.example.com when geting network status

Because sometimes dns won't return for www.google.com in China,
ridiculous enough, right?

frontend/ui/networkmgr.lua

@@ -82,7 +82,7 @@ end
 
 function NetworkMgr:getWifiStatus()
     local socket = require("socket")
-    return socket.dns.toip("www.google.com") ~= nil
+    return socket.dns.toip("www.example.com") ~= nil
 end
 
 function NetworkMgr:setHTTPProxy(proxy)