Added legit frequency x-axis & new class to handle all plotting functionality

Finally created a legit frequency x-axis using an
XYMultipleSeriesDataset & a builder method.

The code I based my code off of was from the AChartEngine Demo
application, specially the TrigometricFunctions activity.

2 List<double[]>'s were needed:
 - one for the xAxis values (frequency axis)
 - one for the y-values (FFT data)

Since all I needed (for now) to plot was one series, or one array of
FFT data, I only made the List<>'s (1) element long.
--> This element is an array of doubles, which contains the FFT data.

The dataset builder method calls the addXYSeries method, which maps
each x-axis value to its respective value in the FFT data array.

This was tested and proven to work with various sizes of FFT data.

One-by-one, I moved the rest of the AChartEngine code used in my
MainActivity to a new class called PlotSettings. Now, all chart
settings, dataset builders, rendering, and anything regarding plotting
& chart appearance is located in this new class. So far its been fully
functional.