ignore initial request to disable iptable chain

iptable child chains are disabled by default.  Ignore the initial
disable request so netd won't throw errors when it cannot detach chains
that do not exist.

Bug: 22800189
Change-Id: I70b4b8b9e6a5dbffe32841941922b7fd92dcd3d6
Original-Change-Id: I9312dd558ba6d59dc6aa394afeb66a222daafcf9
[cernekee: backported from master branch]