ANDREA SHALAL and CARIEN DUPLESSIS U.S. Treasury Secretary Janet Yellen's three-country trip to Africa - the leading edge of a new diplomatic push by the Biden administration - aims to show the continent the United States is a true partner, one here for the long haul. But after decades of losing ground to China and the tumult of the Donald Trump years, when the former president threatened to slash aid and roll back military support, it is a tough sell. As Africa struggles with economic headwinds caused by the COVID-19 pandemic, the war in Ukraine and, notably, Washington's own monetary…