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