Use UnknownHostException directly without casting it

ExecutionException cannot be casted to UnknownHostException. If
an unresolvable dns server is set, the ExecutionException will be
triggered and cause cast exception.

Bug: 123435238
Test: atest com.android.server.connectivity.NetworkMonitorTest
Test: Manual test with setting an unresolvable hostname in private
      dns config
Change-Id: Icdbc33ddd03740043cac2eff7dec209ffb500e2a