2,9b-trans-3,9b-cis-2,3-Bis(4-bromo­phenyl)-2,3,3a,4,5,9b,1′,2′,3′,4′-deca­hydro-1H-benzo­[e]­indene-1-spiro-2′-naphthalene-1′,9b-diol acetone solvate