dnsmasq: Update listeners when interface set changes

dnsmasq does not update its list of listeners with the new
interface object whenver an interface goes up/down and hence
when we free the old interface objects, the interface info in
dnsmasq listeners is lost. This fix makes sure the listenerss
are updated with the new interface object

Change-Id: Ib3204d0d94c28a57f473fc04c5a10a52fc48e754
CRs-Fixed: 586287
dnsmasq: dnsmasq should not die if interface not found

When an instantaneous tether and untether command are issued
the dnsmasq operation of tether happens after the interface
has been brought down by the untether command hence
tethering goes into bad state. The fix is to let dnsmasq
running so that it can be killed later by the untether command.

CRs-Fixed: 620556
Change-Id: Ia92f530d2d082758bcefd1c4728bb21edcc39043