LONDON — It has been five years since Shirwa Ahmed, a 26-year-old from Minneapolis, blew himself up in northern Somalia, and sent shivers up American spines about young immigrants from war-torn Muslim countries who were turning into terrorists. Unconfirmed reports that American Muslims might have been among the gunmen who stormed Nairobi’s Westgate shopping mall over the weekend will no doubt revive fears of the “enemy within.” [...] I remain, however, unafraid of Somalis, least of all of Americanized ones. I spent time in Minnesota in 2011 — the Twin Cities is home to the greatest concentration in the United...