Amazon will boost staffing at its secretive Silicon Valley-based hardware unit by at least 27 percent over the next five years as it tests Internet-connected “smart” home gadgets such as a one-button device to order supplies. The plans, detailed in a little-known government document and by people familiar with the matter, signal Chief Executive Officer Jeff Bezos’s intentions to double down on the No. 1 U.S. online retailer’s hardware strategy. This is despite the lukewarm reception of Amazon’s new Fire smart phone and some investors’ criticism of its heavy spending on highly experimental projects. The Lab126 division, which developed Amazon’s Kindle and other consumer electronics devices, plans to boost its full-time payroll to at least 3,757 people by 2019, according to the agreement reached with California in June that would give Amazon $1.2 million in tax breaks.