Add missing DevSdkIgnoreRule rule variable

DevSdkIgnoreRule.IgnoreUpTo() will not work as expected without
the rule defined.

Bug: 184750836
Test: atest android.net.Ikev2VpnProfileTest#\
      testBuildExcludeLocalRoutesSet works as expected with ignore
      rule
Change-Id: I03a5b28963cd6feebe4358d57c070d941cd114e5

tests/unit/java/android/net/Ikev2VpnProfileTest.java

@@ -35,6 +35,7 @@ import com.android.testutils.DevSdkIgnoreRule;
 import com.android.testutils.DevSdkIgnoreRunner;
 
 import org.junit.Before;
+import org.junit.Rule;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
@@ -64,6 +65,9 @@ public class Ikev2VpnProfileTest {
     private static final byte[] PSK_BYTES = "preSharedKey".getBytes();
     private static final int TEST_MTU = 1300;
 
+    @Rule
+    public final DevSdkIgnoreRule ignoreRule = new DevSdkIgnoreRule();
+
     private final MockContext mMockContext =
             new MockContext() {
                 @Override