Selectively add stats based on iface active state.

Only combine /proc/net/dev and xt_qtaguid stats when iface is marked
as active.  When inactive, only return xt_qtaguid stats.  When iface
is unknown to xt_qtaguid, always pass through /proc/net/dev stats.

Bug: 5242730
Change-Id: I469fc6abe45309f794afebca814cbb39e4f13af5