Use iptables pipes when fetching tether counters.

Tested using:

adb shell ndc bandwidth gettetherstats
adb shell iptables -nvx -L natctrl_tether_counters
adb shell ip6tables -nvx -L natctrl_tether_counters

Results:

114 0 wlan0 rmnet_data0 272883 2976 8624804 6032
200 0 Tethering stats list completed
Chain natctrl_tether_counters (2 references)
    pkts      bytes target     prot opt in     out     source destination
    2688   179096 RETURN     all  --  wlan0  rmnet_data0  0.0.0.0/0 0.0.0.0/0
    5713  8351999 RETURN     all  --  rmnet_data0 wlan0   0.0.0.0/0 0.0.0.0/0
Chain natctrl_tether_counters (1 references)
    pkts      bytes target     prot opt in     out     source destination
     288    93787 RETURN     all      wlan0  rmnet_data0  ::/0 ::/0
     319   272805 RETURN     all      rmnet_data0 wlan0   ::/0 ::/0

Test: manual test described above
Test: data usage increases by 10MB when downloading 10MB file
Test: netd_unit_test passes
Bug: 34873832
Change-Id: I32c4e750a4d3c379074cc13ab1302d51421860d2