Add startHandshake to IDnsTlsSocket

This is a refactor change which separates the handshake code from
DnsTlsSocket::initialize(). The plan is that initialize() will
continue running on query threads but the code for connection
handshake will run on either query threads or loop threads depending
on a flag.

Bug: 149445907
Test: cd packages/modules/DnsResolver && atest
Change-Id: I262f978230fb1a01ca7963de03b64cb439a37eec