Fix NullPointerException in Ikev2VpnTest for watches

Watches do not have a VpnManager instance anymore, so the tests crashes
at the teardown step when trying to interact with a null VpnManager.

Bug: 368624224
Bug: 340928692
Test: Ikev2VpnTest
Change-Id: Iedf784625933e73bcb5fed9bfa36fbae1e11d398
diff --git a/tests/cts/net/src/android/net/cts/Ikev2VpnTest.java b/tests/cts/net/src/android/net/cts/Ikev2VpnTest.java
index f6a025a..cb55bd5 100644
--- a/tests/cts/net/src/android/net/cts/Ikev2VpnTest.java
+++ b/tests/cts/net/src/android/net/cts/Ikev2VpnTest.java
@@ -184,6 +184,8 @@
 
     // Static state to reduce setup/teardown
     private static final Context sContext = InstrumentationRegistry.getContext();
+    private static boolean sIsWatch =
+                sContext.getPackageManager().hasSystemFeature(PackageManager.FEATURE_WATCH);
     private static final ConnectivityManager sCM =
             (ConnectivityManager) sContext.getSystemService(Context.CONNECTIVITY_SERVICE);
     private static final VpnManager sVpnMgr =
@@ -205,12 +207,15 @@
 
     @Before
     public void setUp() {
-        assumeFalse("Skipping test because watches don't support VPN",
-            sContext.getPackageManager().hasSystemFeature(PackageManager.FEATURE_WATCH));
+        assumeFalse("Skipping test because watches don't support VPN", sIsWatch);
     }
 
     @After
     public void tearDown() {
+        if (sIsWatch) {
+            return; // Tests are skipped for watches.
+        }
+
         for (TestableNetworkCallback callback : mCallbacksToUnregister) {
             sCM.unregisterNetworkCallback(callback);
         }