DoH: Send pending data out right after handle a buffered DNS request

After handling a buffered DNS request, the DnsResolver doesn't
send the DoH query out to the server immediatly. Instead, it
starts waiting for any new DNS request or any packets from the
server, which can lead to the DNS request timed out.

So this change makes the DnsResolver send any pending data
immediately after it handles a buffered DNS request.

Bug: 275539642
Test: ran resolv_integration_test.PrivateDnsDohTest#RunOutOfStreams
      2000 times
Change-Id: I4386a929618a251e3b92fe4724fdfe85e0c44bd4