mod: 优化判断是否往上滑动

[xymelon]

@jeasonlzy 我改了下HeaderScrollHelper isTop的判定，你验证下？

[xymelon]

统一使用这个判定

/**
     * 判断当前view是否往上滑动
     *
     * @param view
     * @return
     */
    private boolean canViewScrollUp(View view) {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            if (view instanceof AbsListView) {
                final AbsListView absListView = (AbsListView) view;
                return absListView.getChildCount() > 0
                        && (absListView.getFirstVisiblePosition() > 0 ||
                        absListView.getChildAt(0).getTop() < absListView.getPaddingTop());
            } else {
                return ViewCompat.canScrollVertically(view, -1) || view.getScrollY() > 0;
            }
        } else {
            return ViewCompat.canScrollVertically(view, -1);
        }
    }

[wanghainan]

PagerSlidingTabStrip，三个title怎么让他们默认居中啊