WASHINGTON — It was a long day in Angola. President Biden had already visited a port facility bracketed with cranes and toured a factory filled with conveyor belts. So by the time he sat down at a large wooden circular table in a warm, stuffy room with African leaders, he put his head in his hand and briefly closed his eyes as the speeches droned on. Flying across the world would have tired even a president younger than 82. But the point, as he…