PercentageCircle 自定义圆环效果，指定中心园的背景颜色，圆环颜色，文字颜色，以及中心园的半径。
主要实现：
1.中间的圆 2.外边的圆环 3.中间的文字
//设置中心园的画笔
mCirclePaint = new Paint();
mCirclePaint.setAntiAlias(true);
mCirclePaint.setColor(mCircleBackground);
mCirclePaint.setStyle(Paint.Style.FILL);
//设置文字的画笔
mTextPaint = new Paint();
mTextPaint.setColor(mTextColor);
mTextPaint.setAntiAlias(true);
mTextPaint.setStyle(Paint.Style.FILL);
mTextPaint.setStrokeWidth((float) (0.025*mRadius));
mTextPaint.setTextSize(mRadius/2);
mTextPaint.setTextAlign(Align.CENTER);
//设置外圆环的画笔
mArcPaint = new Paint();
mArcPaint.setAntiAlias(true);
mArcPaint.setColor(mRingColor);
mArcPaint.setStyle(Paint.Style.STROKE);
mArcPaint.setStrokeWidth((float) (0.075*mRadius));
