Fix tethering in the case of a regular upstream connection.

Fixes tethering via Ethernet, Bluetooth and WiFi (hotspot).

Tethering when the upstream has a DUN-specific APN is likely still broken
(untested).

For now, assign a fixed NetId (a hack) until we can change the framework to
create a valid NetworkAgent and all that jazz.

Bug: 15968336
Bug: 14988803
Change-Id: Idcf4d492d9329a9c87913e27be6dd835a792bea2