Tchaikovsky called Mozart the greatest of all composers, and this concert is an audience-pleasing convergence of the two. Jeffrey Kahane is both conductor and soloist in Mozart’s graceful 17th Piano Concerto, and cello virtuoso Alisa Weilerstein joins the Philharmonic for Tchaikovsky’s Rococo Variations, a kind of tribute to Mozart that extends from ravishing to rhapsodic to bravura.