Add StackedLinkProperties test case

Change access level of getNetworkAgentInfoForNetwork in ConnectivityService to get
better testability. And add test case to verify clat state transition and make sure
linkProperties are updated correctly.

Bug: 80261579, 109913107
Test: runtest frameworks-net -c com.android.server.ConnectivityServiceTest \
          -m testStackedLinkProperties
Change-Id: I72fce594d74bd349f65557edca03640e1b86317c
diff --git a/services/core/java/com/android/server/ConnectivityService.java b/services/core/java/com/android/server/ConnectivityService.java
index 69b2bbf..49d60a7 100644
--- a/services/core/java/com/android/server/ConnectivityService.java
+++ b/services/core/java/com/android/server/ConnectivityService.java
@@ -990,7 +990,8 @@
         }
     }
 
-    private NetworkAgentInfo getNetworkAgentInfoForNetwork(Network network) {
+    @VisibleForTesting
+    protected NetworkAgentInfo getNetworkAgentInfoForNetwork(Network network) {
         if (network == null) {
             return null;
         }