WFLY-3387 Make sure JSF managed properties are injected before @PostConstruct methods are invoked

Call WebInjectionContainer#newInstance from MyFacesLifecycleProvider#postConstruct instead of from MyFacesLifecycleProvider#newInstance to make sure that JSF managed properties get injected before @PostConstruct methods are invoked when using MyFaces. 

More details in https://issues.jboss.org/browse/WFLY-3387
Thanks for catching this, Stuart. It doesn't look like MyFaces already has its own doPrivileged block so I've now wrapped this.