loader: discover the ping URL using network service discovery

This is currently a bit hacky. The Android API is quite incomplete for
that purpose and therefore, we need to rely on service name to detect
our service. See:
 https://code.google.com/p/android/issues/detail?id=35810

README has been updated to document how the service should be setup.
loader: wait longer and react sooner for network service discovery

We wait up to 10 seconds for network discovery to get the appropriate
service. But we also react sooner if the service is found.