In a matter of hours this week, the Trump administration twice weighed in on one of the central issues shaping business and society today — just how much market power big companies should be allowed to amass. Yet in back-to-back developments, two federal agencies arrived at starkly different conclusions, and one company, AT&T, found itself on opposite sides of the debate. On Monday, the Department of Justice sued to block AT&T’s proposed $85.4 billion takeover of Time Warner, a deal that would unite one of the country’s biggest Internet providers with the company that owns CNN, HBO and the Warner Bros.