Implement DNS probe in DoT validation

The purpose of this change is to avoid using DoT servers if they
are much slower than DNS servers. The mechanics is flag-guarded,
and it performs in DoT validation.

The mechanics works as follows:
1. Make use of the original DoT query to establish the connection.
2. Use the same DNS packet to issue a DoT query in parallel with a
   UDP query to the same DoT server.
3. If UDP query failed or is lost, issue another one.
4. See the latencies of both queries. Decide if DoT validation
   can pass.

DoT validation passes if dot_latency is less than (a * udp_latency + b),
where a and b are configurable by dot_validation_latency_factor and
dot_validation_latency_offset_ms respectively.

Bug: 188153519
Test: run resolv_integration_test twice
Test: run atest with all the flags off/on
	dot_validation_latency_factor: -1 / 3
	dot_validation_latency_offset_ms: -1 / 100
        sort_nameservers: 0 / 1
        dot_xport_unusable_threshold: -1 / 20
        dot_query_timeout_ms: -1 / 10000
        keep_listening_udp: 0 / 1
        parallel_lookup_sleep_time: 2 / 2
        dot_revalidation_threshold: -1 / 10
        dot_async_handshake: 0 / 1
        dot_maxtries: 3 / 1
        dot_connect_timeout_ms: 127000 / 10000
        parallel_lookup_release: UNSET / UNSET

Change-Id: I8507c409b0cb6e48655d54611256917392db69ac