DoH: Handle connection draining state

When a CONNECTION_CLOSE frame is received, the connection enters
draining state (RFC 9000 section 10.2.2). When in draining state,
no more packets can be sent. So the DoH client needs to initiate
a new connection if necessary; otherwise, subsequent DNS requests
might fail.

As a connection enters draining state upon a CONNECTION_CLOSE
frame is received, DNS packets sent prior to the frame received
might fail. For example, in Quiche (version 0.9.0) implementation,
the DoH client can send some DNS packets out on the same
connection after predefined max_idle_timeout seconds (55 seconds)
because of large Probe Timeout (RFC 9002 section 6.2). It might
lead to the DoH server responding with a CONNECTION_CLOSE frame,
and result in those DNS queries failed.

Bug: 215818810
Test: cd packages/modules/DnsResolver && atest
Test: manual test (see bug 215818810 comment#2)
      verified that the DoH client can create a new connection for
      subsequent DNS requests after detecting a connection being
      draining state.
Change-Id: Ibd1667aeb9e1528776392e6f91ddb13b66637f33