In a lab shut off from communication with the outside world and where visitors can’t bring in a pad of paper, let alone a phone, Apple Inc. has given some companies special early access to Apple Watch. Bayerische Motoren Werke AG, Facebook Inc., United Continental Holdings Inc. and others have spent weeks at Apple’s headquarters in Cupertino, California, working hands-on with the smartwatch to test and fine-tune applications that will debut alongside the device next month, according to people familiar with the process, who asked not to be identified because of non-disclosure agreements.