wifi: show devices behind a firewall that are connected to the  WiFi hotspot

* Clients that are behind a firewall, in particular Windows-PCs tend
to not respond to the request invoked by the isReached(val)-method.
Executing a ping instead and evaluating the outcome enables us to
bypass the firewall.

Change-Id: If857545c7a4d5fbc9d2dc919096a9c84e64d6877
Only start one WifiClientScanner

*) Fixes unexpected soft reboots.
E/AndroidRuntime(18586): *** FATAL EXCEPTION IN SYSTEM PROCESS: WifiClientScanner
E/AndroidRuntime(18586): java.lang.IllegalStateException: Cannot execute task: the task is already running.
E/AndroidRuntime(18586):     at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:576)
E/AndroidRuntime(18586):     at android.os.AsyncTask.execute(AsyncTask.java:535)
E/AndroidRuntime(18586):     at com.android.server.connectivity.Tethering$WifiClientScanner.handleMessage(Tethering.java:596)
E/AndroidRuntime(18586):     at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime(18586):     at android.os.Looper.loop(Looper.java:136)
E/AndroidRuntime(18586):     at android.os.HandlerThread.run(HandlerThread.java:61)

*) Remove unnecessary mDoScan class member variable.

Change-Id: I17ab368366b9a3456a625974c461783c428efec2