Improve thread safety (#12395)

See: https://vaadin.com/forum/thread/17522264/concurrentmodificationexception-in-vaadin-shared-on-karaf-4-2-x

shared/src/main/java/com/vaadin/osgi/resources/impl/VaadinResourceTrackerComponent.java

@@ -151,9 +151,11 @@ public class VaadinResourceTrackerComponent {
      */
     @Activate
     protected void activate(BundleContext context) {
-        vaadinSharedContext = context;
-        for (Delegate<?> registration : resourceToRegistration.values()) {
-            registration.register(vaadinSharedContext, vaadinService);
+        synchronized (resourceToRegistration) {
+            vaadinSharedContext = context;
+            for (Delegate<?> registration : resourceToRegistration.values()) {
+                registration.register(context, vaadinService);
+            }
         }
     }