Well finally found solution for this. Trick was to use capability as NET_CAPABILITY_INTERNET. Which is same as startUsingNetworkFeature(ConnectivityManager.TYPE_MOBILE, FEATURE_ENABLE_HIPRI);
See the Firmware design here
builder.addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET);
builder.addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR);
After this I am able to get onAvailable callback from system and later I set my process default network as mobile data.
Hence all the request goes over mobile data even if wifi is on. WOW!
Note: This was not working in initial releases of Preview L.
Edit 19-10-2015: setProcessDefaultNetwork is now depcreated use bindProcessToNetwork