Fix DnsResolver log print

When mDNS queries/send_dg() are timed out, the log prints out
"E resolv  : send_mdns: 1" which is not meaningful.

This commit changes the log print to "E resolv : send_mdns: timeout"
to provide an obvious message

Bug: 209492138
Test: checked the DnsResolver log
Change-Id: I07de9c4d2f10139debc3c9e76e625af616b9e076