AbstractSingletonService: fix helper method

The filterInstances method should not alter the return type of the input
list. The purpose of that method is to potentially remove some elements
from the input list -- nothing more -- so it should still return a
List<PT>, not something else.

Author: hinerm

src/main/java/org/scijava/platform/DefaultPlatformService.java

@@ -134,7 +134,7 @@ public boolean registerAppMenus(final Object menus) {
 	}
 
 	@Override
-	public List<? extends Platform> filterInstances(final List<Platform> list) {
+	public List<Platform> filterInstances(final List<Platform> list) {
 		final Iterator<Platform> iter = list.iterator();
 		while (iter.hasNext()) {
 			if (!iter.next().isTarget()) {

src/main/java/org/scijava/plugin/AbstractSingletonService.java

@@ -88,7 +88,7 @@ public void initialize() {
 		objectService.getIndex().addLater(new LazyObjects<PT>() {
 
 			@Override
-			public ArrayList<PT> get() {
+			public List<PT> get() {
 				return new ArrayList<PT>(getInstances());
 			}
 
@@ -108,7 +108,7 @@ public Class<?> getType() {
 	 * @param list the initial list of instances
 	 * @return the filtered list of instances
 	 */
-	protected List<? extends PT> filterInstances(final List<PT> list) {
+	protected List<PT> filterInstances(final List<PT> list) {
 		return list;
 	}