Let resolv_integration_test listen to onPrivateDnsValidationEvent

The resolver integration tests used to consider the private dns validation
complete by means of monitoring whether the testing servers receive the
validation packet or not, which could cause the tests to flake.

The reason is: the validation flag hasn't yet been marked as SUCCESS in
PrivateDnsConfiguration. At that moment, since the testing servers has
received the validation packet, the tests consider the private dns server
is available and then start querying. If the flag still hasn't been marked
as SUCCESS, the query go over udp.

This change make the tests listen to onPrivateDnsValidationEvent which
guarantees the flag has already been marked. Besides, we are able to
have test coverage for onPrivateDnsValidationEvent now.

Bug: 124422785
Bug: 139327133
Test: atest --include-subdirs packages/modules/DnsResolver
Test: resolv_integration_test --gtest_repeat=1000 --gtest_filter="ResolverTest.BrokenEdns"
Change-Id: I0b11f98c3e8c44e4b347621a79815a19689f155b