After pioneering online flash sales in China to reach the top of the smartphone market, Xiaomi Corp. is turning to old-fashioned retail to arrest its slide. The phonemaker will roll out a chain of about 1,000 brick-and-mortar stores under the Mi Home banner over the next three years, as co-founder Lei Jun mimics a strategy that’s helped the Oppo and Vivo brands leapfrog Xiaomi to the top of China’s smartphone market.