Extend DnsTlsServer to store validation state

This change also fixes a bug in PrivateDnsConfiguration which DoT
servers not valid for the network might somehow be counted as
validated servers. For instance, if a validation for DoT server
finishes after the server is removed, the server is mistakenly
deemed as a validated server.

Bug: 79727473
Test: cd packages/modules/DnsResolver && atest
Change-Id: Idee1f34f59dce1451b7b3e87fd20e6795af883ba