Visualizer: replace the FFT implementation with a faster one.

This implementation uses fixed points instead of floating points. It
is slightly inaccurate compared to the old one but still perfect for
visualization purpose. It runs 40% faster on passion, 5 times faster
on sholes, and of course 14 times faster on sapphire.

Change-Id: I1e868417bcffda091becf106a7b941d02813faec