Fix race condition caused by fd reused for DnsResolver

There might be a gap between fd close and fd event listener unregister.
If fd is reused for another query during that gap, it might cause the
query failed with no response since addOnFileDescriptorEventListener
method failed. To fix this problem, we must ensure that fd event
listener is unregistered before fd closing.

Bug: 134310704
Test: atest DnsResolverTest
Change-Id: Ifb6bd34dc54dcf1c61fe8b87785124df4bc0f410