netd: BandwidthController: support reading out tethering stats

* Add
  ndc bandwidth gettetherstats <ifaceIn> <ifaceOut>
which returns
  221  ifaceIn ifaceOut rx_bytes rx_packets tx_bytes tx_packets
If the iface pair is not found it will fail.

221 is the new response code for TetheringStatsResult.
It gets the stats by looking at the iptables FORWARD chain's counters.

* Fixed return handling after some of the responses.
  - no need for errorno
  - after ResponseCode >= 200, don't return another.

* Correctly initialize the alert values on "bandwidth enable"

Bug: 5244846,5230066
Change-Id: I81c941441525fa4055ae270d5cad05e6c42b8f72