Remove usage of hidden .netId in IpConnectivityLog

Use the Network#getNetId API instead.

IpConnectivityLog is planned to be outside of the connectivity module,
so it cannot use hidden symbols implemented in the module.

Bug: 174436414
Test: m
Change-Id: I947a32612c59b53f22fe1c38706de8919ca5043a

core/java/android/net/metrics/IpConnectivityLog.java

@@ -137,7 +137,7 @@ public class IpConnectivityLog {
      * @return true if the event was successfully logged.
      */
     public boolean log(@NonNull Network network, @NonNull int[] transports, @NonNull Event data) {
-        return log(network.netId, transports, data);
+        return log(network.getNetId(), transports, data);
     }
 
     /**