Don't return an unmod collection.

It's not needed as people can mess with this, and people do, so this fixes the build.
bug:8398355

Change-Id: Iddd00af41009c88b67fb7f9a8789bb416c377922

core/java/android/net/LinkProperties.java

@@ -197,7 +197,7 @@ public class LinkProperties implements Parcelable {
         for (LinkProperties stacked: mStackedLinks.values()) {
             routes.addAll(stacked.getAllRoutes());
         }
-        return Collections.unmodifiableCollection(routes);
+        return routes;
     }
 
     public void setHttpProxy(ProxyProperties proxy) {