Terminate private DNS validation threads if network is down

Notify private DNS validations to finish when the associated
network ID is removed. As we always destroy the network before
a test ends, this change is helpful especially for tests because
it ensures that private DNS validations in each test will finish
as soon as the test finishes.

Bug: 232490992
Bug: 233713784
Test: atest
Change-Id: If3585589b33c0d09373fef08884de591e09c5fce