Expose netId by adding getter API

Bug: 151156820
Test: m
      atest TetetheringTests
Change-Id: Ieb1483c146aa2f7d8f251157e6e81d71c44ae899
(cherry picked from commit 8831c4b54178753013d568c0b76bbf7487db8122)
diff --git a/Tethering/src/com/android/server/connectivity/tethering/Tethering.java b/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
index 3d8dbab..3d25e54 100644
--- a/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
+++ b/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
@@ -1458,7 +1458,7 @@
             } else {
                 dnsServers = mConfig.defaultIPv4DNS;
             }
-            final int netId = (network != null) ? network.netId : NETID_UNSET;
+            final int netId = (network != null) ? network.getNetId() : NETID_UNSET;
             try {
                 mNetd.tetherDnsSet(netId, dnsServers);
                 mLog.log(String.format(