A florist chain in Argentina, a food delivery service in Hong Kong, and a Singaporean travel agency—these are a few of the companies relying on a Kenyan startup to help them talk to their customers on WhatsApp, WeChat, and other messaging apps. Ongair, a Nairobi-based startup, says instant messaging could and should replace the traditional channels of customer service—frustrating phone calls, inefficient e-mail exchanges, online chats that don’t work well on a smartphone, or SMS…