Fix replying to queries via unicast

Instead of (wrongly) looking at the DNS message flags for the Query
Unicast bit, look at the flag in the question record.

The previous bug means that all queries would be replied multicast
instead of sometimes unicast. The fix is flagged off as this is a
behavior change that may affect performance and latency.

Bug: 289482497
Test: atest
Change-Id: I08e30c4ffa747c9073d631e8addca1278ea40648