Shell: Grant NETWORK_STACK permission for CTS

Grant NETWORK_STACK permission to the shell so
that CTS can use the shell identity to call
APIs protected with this permission.

Bug: 150236894
Test: atest android.net.wifi.cts.WifiManagerTest#testGetCountryCode
Change-Id: I4f93e76bcda83c528f1f2c7bcf800965886ea8da