Add new NetworkType for WIFI+CELLULAR+VPN transport

Define NT_WIFI_CELLULAR_VPN in enum NetworkType.
=>Indicates this network uses a WIFI+CELLULAR+VPN transport.

Bug: 143732914
Test: Manual test with statsd_testdrive
Merged-In: I1d43633705b69bf0111386630b2eebf394cb9366
Change-Id: I4575b912db7457e0e0c1dea4d1af07389a4cf394
(cherry picked from commit b1997b78)