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

Change-Id: I260a68c1f9cfa7354730b7a551cce0a66e645db1