Fix wrong behavior of async DNS API with flags

Bug: 132323069
Test: built, flashed, booted
      system/netd/tests/runtests.sh pass
      atest MultinetworkApiTest#testResNApi
      atest DnsResolverTest
      atest CtsNativeNetDnsTestCases

Merged-In: I99f1f17986774cf34e1361719b4322586d0e081f
(cherry picked from commit 80d0cbe9a181207e9ceb4d6b94ba799a39058776)

Change-Id: Id3d67d345657810fc79fb683e80aced6c518a382