fix Java9 API usage in Java8 module

Author: thjaeckle

policies/model/src/test/java/org/eclipse/ditto/policies/model/ImmutableImportsAliasTest.java

@@ -15,7 +15,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
-import java.util.List;
+import java.util.Collections;
 
 import org.eclipse.ditto.json.JsonFactory;
 import org.eclipse.ditto.json.JsonObject;
@@ -119,7 +119,7 @@ public void fromJsonWithEmptyTargetsArraySucceeds() {
     @Test
     public void roundTripJsonSerialization() {
         final ImportsAlias original = ImmutableImportsAlias.of(LABEL,
-                List.of(ImmutableImportsAliasTarget.of(IMPORTED_POLICY_ID, ENTRY_LABEL)));
+                Collections.singletonList(ImmutableImportsAliasTarget.of(IMPORTED_POLICY_ID, ENTRY_LABEL)));
 
         final JsonObject json = original.toJson();
         final ImportsAlias deserialized = ImmutableImportsAlias.fromJson(LABEL, json);