For decades, Walmart has attracted attention, and taken heat, for how it treats its workforce, including paying low wages and creating unpredictable schedules. Now, the giant retailer is teaming up with an Oakland tech company to try to ease some of its workers’ financial strain. The nation’s largest private employer has started providing its more than 1.4 million workers a service that will allow them to receive wages before their next payday. Instead of waiting two weeks between paychecks, Walmart workers can now use an app to access a portion of wages for hours they have already worked.