Implement DoT revalidation

The revalidation starts from DnsTlsDispatcher which uses a counter
for counting the number of continuous network_error failures of a
DoT server. The mechanics works for private DNS opportunistic mode.

- Once the counter reaches dot_revalidation_threshold, DnsTlsDispatcher
  sends a revalidation request to PrivateDnsConfiguration to validate
  the DoT server.
- Once the counter reaches dot_xport_unusable_threshold, DnsTlsDispatcher
  marks the transport of the DoT server as unusable. The DoT server
  won't be used for at least 5 minutes.

DoT revalidation runs when the followings are met:
  [1] the private DNS setting is opportunistic mode
  [2] the requested DoT server is valid to be used on the network
  [3] the requested DoT server is currently marked as Validation::success

The above mechanics runs when the feature flag "dot_revalidation_threshold"
is a positive and zon-zero value, and is -1 when the mechanics is
disabled.

Bug: 79727473
Test: atest when all the flags off
        dot_revalidation_threshold: -1
        dot_async_handshake: 0
        dot_xport_unusable_threshold: -1
        dot_maxtries: 3
        parallel_lookup_sleep_time: 2
        dot_connect_timeout_ms: 127000
        parallel_lookup_release: 0
        sort_nameservers: 0
        keep_listening_udp: 0

Test: atest when all the flags on
        dot_revalidation_threshold: 10
        dot_async_handshake: 1
        dot_xport_unusable_threshold: 20
        dot_maxtries: 1
        parallel_lookup_sleep_time: 2
        dot_connect_timeout_ms: 10000
        parallel_lookup_release: 1
        sort_nameservers: 1
        keep_listening_udp: 1

Change-Id: Id442529468d63156a9aebf30ea5f142dfa689a97