使FrameLayout的Gravity等于Center

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由于无法设置FrameLayout的Gravity，所以只能通过重写onLayout事件实现居中的效果了

@Override
protected void onLayout(boolean changed, int left, int top, int right,
		int bottom) {

	super.onLayout(changed, left, top, right, bottom);

	// align child to center
	int width = right - left;
	int height = bottom - top;

	final int childCount = getChildCount();
	for (int i = 0; i < childCount; i++) {
		final View childView = getChildAt(i);

		if (childView.getVisibility() != View.GONE) {
			final int childLeft = (width - childView.getWidth()) / 2;
			final int childTop = (height - childView.getHeight()) / 2;

			childView.layout(childLeft, childTop,
					childLeft + childView.getWidth(),
					childTop + childView.getHeight());
		}
	}
}