dnsmasq: Fix issues observed while toggling tethering

- 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 and hence
  tethering goes into bad state. The fix is to leave dnsmasq
  running so that it can be killed later by the untether command.
- 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 listeners
  are updated with the new interface object

CRs-fixed: 665103
Change-Id: I0322619edc12146c67fa7a308769e3b9f0779654
dnsmasq: Revert change for updating listeners when interface set changes.

This is now handled by the function
fixup_possible_existing_listener

Change-Id: Iedde8317b619830a494f7d1d82b3da7a6c6ac9e5