Allow to do TLS handshake on DnsTlsSocket loop thread

There are some performance concerns about running connection handshake
on query threads (which are launched from DnsProxyListener):
  - Until a handshake running on a query thread finishes, other
    query threads get blocked from acquiring the lock, which
    is hard to implement timeout.
  - If the handshake fails, all of the waiting query threads can't
    know it. Then, one of the them will do another handshake which
    is likely to fail again.

This change introduces a flag which moves connection handshake from
query threads to DnsTlsSocket loop thread to address the concerns.
  - Before a handshake finishes, query threads are waiting their
    std::future result. This helps future implementation for query
    timeout.
  - If the handshake fails kMaxTries times, none of the waiting query
    threads does another handshake to the same DoT server again.
    They can either try next DoT server or falls back to Do53.

Besides, with the flag enabled, DnsTlsSocket can be aware of
shutdown request and instantly stop connection handshake if needed.

Bug: 149445907
Test: cd packages/modules/DnsResolver
      set the flag on, ran atest
      rebooted, set the flag off, ran atest
Change-Id: I6a0a4c962c9eaf78ca2796128018f7ba06897b16