Apply PrivateDnsValidationObserver to DnsTlsDispatcher

Also aligh the life time of DnsTlsDispatcher with
PrivateDnsConfiguration's so that the DnsTlsDispatcher won't miss
any notification.

Bug: 79727473
Test: cd packages/modules/DnsResolver && atest with the flag on/off
Change-Id: Iaf1c809b79373b6846e6535aa6f0c619f430898c