BandwidthController: hookup qtaguid for tracking closest to devices.

Before, qtaguid would rely on netdev (up/down) callbacks to read out the
dev rx/tx stats.
Now it uses (raw) PREROUTING and (mangle) POSTROUTING iptable hooks.
We use mangle on the out path because nat is not available in ipv6.
The NatController only masquerades in nat POSTROUTING so we don't count
traffic that might get dropped.


Bug: 5444089
Change-Id: I997b8b3c81264a545f1f575baaf0445c0afc8f72