Revise calling flow of OnDnsEvent and drop 'struct'

1. Drop 'struct' in DnsProxyListener
2. Revise calling flow of OnDnsEvent
3. Add event define in INetdEventListener
   (EVENT_GETHOSTBYADDR/EVENT_RES_NSEND)
4. Remove unsed mNetdEventListener

Test: built, flashed, booted
      system/netd/tests/runtests.sh passes

Change-Id: I687c807c3d41c47349078db4240eb63f6ba658ac