(Fair Labor Standards Act (FLSA) action filed by class of aluminum factory workers contending defendant failed to compensate them for time spent arriving early for shift relief, donning and doffing, walking from locker room to work site and showering at shift end; defendant filed motion for summary judgment and federal district court granted relief; any walking time that occurs after beginning of employee’s first principal activity is compensable under FLSA, but time spent donning and doffing was not a principal activity because it was not integral to employees’ work).