[Feature sync] Fix flaky test in MdnsSocketClientTests

The interface index is only propagated if there is an A/AAAA
record in the response. Since the responses here are not entirely
controlled in the test, we just want to verify that we get a
response with the target we expect.

Verified by running the test 1400 times via --runs_per_test flag.

Bug: 254155029
Fix: 255922416
Test: atest MdnsSocketClientTests#startDiscovery_andPropagate\
      InterfaceIndex_includesInterfaceIndex --iteration 100
Change-Id: Icb0fe7a58a1e626b7f781ff24c2389a3095d4dc6