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

This reverts part of commit 1604e18615f824adb2d54c36320069ba6fcb4796.
The DNS cache flushing code needs to be called when routes change.

Change-Id: I20d39eabfa088deeeeeff0c3292b1910fd948b0d
bug:16549455
diff --git a/server/ResolverController.cpp b/server/ResolverController.cpp
index 8d807af..a5d69ae 100644
--- a/server/ResolverController.cpp
+++ b/server/ResolverController.cpp
@@ -37,3 +37,13 @@
 
     return 0;
 }
+
+int ResolverController::flushDnsCache(unsigned netId) {
+    if (DBG) {
+        ALOGD("flushDnsCache netId = %u\n", netId);
+    }
+
+    _resolv_flush_cache_for_net(netId);
+
+    return 0;
+}