With its second 7-inch Android device, Google just served notice on the tablet makers of the world. Again. The new Nexus 7 (starting at $229, $269 as tested) sports an even sleeker profile than before, a high-def 1920 x 1200-pixel screen, a powerful processor and stereo speakers. It's also the first device to run Android 4.3. Is all that worth $30 more than the original?