Update and fix Visualizer class docs

1) Fix the formula for the 'kth frequency' in getFft method:
   since the 'fft' array holds n / 2 frequencies ranging from
   0 to Fs / 2, the correct formula is k*Fs/n.
   Also fix the last frequency index in the table.

2) Move the sample code for calculating magnitudes and phases
   from the fft array from 'onFftDataCapture' to 'getFft', provide
   a link.

3) Fix the sample code which wasn't taking into account that
   fft[0] is the real part of FFT at DC, and fft[1] is the real part
   of FFT at Fs / 2. Improve the sample code by linking the variable
   names closer to the variable names in the documentation.

Bug: 113790190
Test: atest CtsMediaTestCases:VisualizerTest
Change-Id: I07cc9cfcda5b8c44aae2742a26173e7a9475120a