Terminate private DNS validation threads if network is down am: 22432351

Original change: https://android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/2106089



Change-Id: I230ce9e91ebd84606115b9feb11f1cc89e4d3dc2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>