Add logs for changes in capabilities and score

This will be useful for diagnosing default network changes.

Logs may look like, for example :
Update capabilities for net 100 : -NET_CAPABILITY_OEM_PAID+NET_CAPABILITY_TRUSTED
Or :
Update score for net 100 : -POLICY_EVER_EVALUATED+POLICY_IS_VPN+POLICY_IS_DESTROYED

Test: FrameworksNetTests
      manual
Change-Id: Ic4788c599573aae9daeca995b8853800aaaba0b8
diff --git a/service/src/com/android/server/ConnectivityService.java b/service/src/com/android/server/ConnectivityService.java
index d409d51..e732001 100755
--- a/service/src/com/android/server/ConnectivityService.java
+++ b/service/src/com/android/server/ConnectivityService.java
@@ -8008,6 +8008,10 @@
             @NonNull final NetworkCapabilities nc) {
         NetworkCapabilities newNc = mixInCapabilities(nai, nc);
         if (Objects.equals(nai.networkCapabilities, newNc)) return;
+        final String differences = newNc.describeCapsDifferencesFrom(nai.networkCapabilities);
+        if (null != differences) {
+            Log.i(TAG, "Update capabilities for net " + nai.network + " : " + differences);
+        }
         updateNetworkPermissions(nai, newNc);
         final NetworkCapabilities prevNc = nai.getAndSetNetworkCapabilities(newNc);