Arrive at the corner of Oakland’s Grand Avenue and MacArthur Boulevard and you will hear the sounds of eclectic music emerging from multiple spots along Lake Merritt. Aaron Davis Warren, otherwise known as “Drummer Boy Aaron,” set up his drum set just before 10 a.m. on a recent Saturday to play for the crowd that comes from all directions. The 18-year-old pounds his drumsticks as he smiles widely at his growing audience. To his right, people carrying mushrooms and cabbage emerge…