Fix private DNS not working due to getConnectCounter() stuck

This call is not necessary to be protected by DnsTlsDispatcher::sLock.
The lock aims at protecting the creation/deletion of DnsTlsTransport.
Since the call is used only for the metrics, this change doesn't impact
on the functionality of private DNS.

Bug description:
When a thread gets stuck in TLS handshake, it might lead to another
thread blocking in the call getConnectCounter(). This results in
other DNS requests get stuck in the call getOrderedServerList()
because they are awaiting a lock hold by the second thread which
is also awaiting another lock hold by the first thread.

An example scenario is: In DNS strict mode, a private DNS server
used to be available in mobile data but becomes unresponsive.
Then, a new DNS request triggers the DnsResolver to do handshaking.
Before the handshake finishes, wifi can't work.

How to reproduce the bug:
  1) Set DNS strict mode, turn on mobile data
  2) Wait 20s for TLS connection disconnected. It can be check by:
       - ps -AT $(pidof netd) | grep "TlsListen"
  3) Drop DoT traffic by the command:
       - iptables -A OUTPUT -p tcp --dport 853 -o rmnet_data1 -j DROP
  4) Turn on wifi. Wifi shows no internet

Bug: 160027328
Test: not reproducible by the above steps
Test: cd packages/modules/DnsResolver && atest

Change-Id: I050ce8f13c19f706d58bac44c0e5a269481cb0c0