Fix memory leak in resolver cache

Memory leaks in resolver cache when network is being destroyed.
We clear DNS servers before deleting the cache to make sure the
cache will not be created again.

Bug: 122940321
Test: built, flashed, booted
      system/netd/tests/runtests.sh passed

Change-Id: I6aacc13ade3ce020b906f8a8a0f870f0741a5a47
diff --git a/server/ResolverController.cpp b/server/ResolverController.cpp
index c746f26..340af6a 100644
--- a/server/ResolverController.cpp
+++ b/server/ResolverController.cpp
@@ -145,6 +145,7 @@
 }
 
 int ResolverController::clearDnsServers(unsigned netId) {
+    // It will create the cache if it doesn't exist.
     RESOLV_STUB.resolv_set_nameservers_for_net(netId, nullptr, 0, "", nullptr);
     if (DBG) {
         ALOGD("clearDnsServers netId = %u\n", netId);