FITBIT SPENT ITS first decade selling activity trackers. With its latest moves, the company is starting to look less like a gear maker selling pricey accessories to fitness buffs and more like a medical-device company, catering to hospitals, patients, and health insurers. The company’s business-to-business arm, called Health Solutions, is now addressing four health conditions—sleep disorders including sleep apnea, diabetes, cardiovascular health and mental health—for employers, health insurers, healthcare providers, and researchers.