WASHINGTON/MEXICO CITY (Reuters) – AT&T Inc agreed to pay $1.7 billion to acquire Mexico’s third-largest wireless operator, Iusacell, as it seeks to grab a slice of a market with lower cellular penetration than the United States and faster potential growth. Mexico’s government earlier this year implemented reforms to shake up its telecom and broadcast market by weakening the dominance of broadcaster Grupo Televisa and billionaire Carlos Slim’s cellphone and fixed-line company America Movil. AT&T is looking to tap into the growing cellphone market in Latin America’s No.