Fix circles inherit alpha (Fixes PhilJay#2620)

ISSUE:
When using multiple LineDataSets like the follows:

    int solidColor = 0xFFFF00FF;
    dataSet.setColor(solidColor);
    dataSet.setCircleColor(solidColor);

    int semiTransparentColor = 0x8A000000;
    fadedSet.setColor(semiTransparentColor);

    LineData data = new LineData(dataSet, fadedSet);

the circles in 'dataSet' will rendered with the alpha from
fadedSet (0x8A).

The reason for this is that mRenderPaint is not reset properly
before drawing the circles. The first time drawCircles is called
the imageCache.fill(...) method is used where the color is set by
mRenderPaint.setColor(set.getCircleColor(i)), restoring the alpha
to 0xFF. The second time homever, imageCache.fill(...) is not called
which means that mRenderPaint will use it's old color/alpha,
which in this case is from fadedSet.

TEST INFO:
To trigger the issue, add the following to LineChartActivity1:

    final ArrayList<Entry> fadedEntries = new ArrayList<>();
    for (int i = 0; i < count; i++) {

        float val = (float) (Math.random() * range) + 3;
        fadedEntries.add(new Entry(i, val));
    }
    final LineDataSet fadedDateSet = new LineDataSet(fadedEntries, "Faded");
    fadedDateSet.setColor(0x42000000);

    dataSets.add(fadedDateSet); // add the datasets

and launch the first item in the example app.

SOLUTION:
This commit replaces mRenderPaint with null when drawing the circle
bitmap. If circleColor has been defined with a semi-transparent
color, it will be drawn that way in the cached bitmap, hence the the
bitmap itself does not need to be drawn with an alpha.

MPChartLib/src/main/java/com/github/mikephil/charting/renderer/LineChartRenderer.java

@@ -652,7 +652,7 @@ protected void drawCircles(Canvas c) {
                 Bitmap circleBitmap = imageCache.getBitmap(j);
 
                 if (circleBitmap != null) {
-                    c.drawBitmap(circleBitmap, mCirclesBuffer[0] - circleRadius, mCirclesBuffer[1] - circleRadius, mRenderPaint);
+                    c.drawBitmap(circleBitmap, mCirclesBuffer[0] - circleRadius, mCirclesBuffer[1] - circleRadius, null);
                 }
             }
         }