DAKAR, Senegal — For the artistic and cultural elites of Senegal, the monthlong Dakar Biennale of Contemporary African Arts is a celebratory moment. For years, “we saw the OFF signs, but we didn’t know what was going on,” said Ndiouga Dia, a 48-year-old leatherworker from Soumbedioune, referring to a series of events organized in parallel to the official government program, …