FELIX-4113 Factories not disposed when the extension provider is leaving

* ServiceTracker.addingService() is expecting a non-null value if interested in the service
 * I was returning null (missed that in refactor), so serviceRemoval() was never called

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1491436 13f79535-47bb-0310-9956-ffa450edef68