Revert DNS cache flush command deletion from "Cleanup: Delete dead code."

This reverts part of commit 7a161c535104555890fa78b99faf1f414fc03e85.
The DNS cache flushing code needs to be called when routes change.

Change-Id: I20d39eabfa088deeeeeff0c3292b1910fd948b0d
bug:16549455

core/java/android/os/INetworkManagementService.aidl

@@ -312,6 +312,11 @@ interface INetworkManagementService
      */
     void setDnsServersForNetwork(int netId, in String[] servers, String domains);
 
+    /**
+     * Flush the DNS cache associated with the specified network.
+     */
+    void flushNetworkDnsCache(int netId);
+
     void setFirewallEnabled(boolean enabled);
     boolean isFirewallEnabled();
     void setFirewallInterfaceRule(String iface, boolean allow);