Despite the arrival of strong competitors such as Apple and Xiaomi, Fitbit has retained the lead in the global wearables market in the second quarter of 2015. The maker of fitness tracking devices shipped 4.4 million units between April and June, an increase of more than 150 percent over last year's June quarter. Fitbit's focus on fitness tracking functionality appears to resonate well with customers who value simplicity over the added functionality of competing devices....