Manage NAT64 prefix discovery lifecycle in the framework.

Currently NAT64 prefix discovery, which runs in netd, is started
by netd itself when a network is programmed with all-IPv6 DNS
servers. Unfortunately this is not correct because in many cases
we program DNS servers before the network is connected and it's
actually possible to send packets to them.

In general netd does not have enough visibility into network
lifecycle management to decide when to start and stop prefix
discovery. So move it into the framework with the rest of the
464xlat control plane.

Bug: 65674744
Test: atest FrameworksNetTests
Change-Id: I8fa051a9c216d9c05082bf7d0bbb0cbd56000162
diff --git a/services/core/java/com/android/server/ConnectivityService.java b/services/core/java/com/android/server/ConnectivityService.java
index 8bd3450..6efa5c1 100644
--- a/services/core/java/com/android/server/ConnectivityService.java
+++ b/services/core/java/com/android/server/ConnectivityService.java
@@ -2908,7 +2908,7 @@
             e.rethrowFromSystemServer();
         }
         mNetworkAgentInfos.remove(nai.messenger);
-        nai.updateClat();
+        nai.clatd.update();
         synchronized (mNetworkForNetId) {
             // Remove the NetworkAgent, but don't mark the netId as
             // available until we've told netd to delete it below.
@@ -5249,8 +5249,8 @@
             synchronized (networkAgent) {
                 networkAgent.linkProperties = newLp;
             }
-            // Start or stop clat accordingly to network state.
-            networkAgent.updateClat();
+            // Start or stop DNS64 detection and 464xlat according to network state.
+            networkAgent.clatd.update();
             notifyIfacesChangedForNetworkStats();
             if (networkAgent.everConnected) {
                 try {