Always file DUN requests for tethering.

A bit of a hack so that first trySetupData call uses the proper APN in
strange edge cases.  Made a new startUsingNetworkFeature string
that ConnectivityService knows means always pass the request to telephony
even if telephony connections are not currently available.

bug:3133178
Change-Id: I2559b9cbaed4b4ec02bdf7ea78e6003ee2be64f2