绘制弧形时简化代码

[OsBelief]

绘制弧形时可以直接在canvas上绘制，不必在bitmap上绘制：
canvas.drawArc(new RectF(circleWidth, circleWidth,
sWidth - circleWidth, sHeight - circleWidth), startAngle,
endAngle, false, mPaint);
并设置画笔mPaint：
mPaint.setStrokeWidth(circleWidth);
mPaint.setStyle(Paint.Style.STROKE);