Fix list divider height is not considered in calculating scrollY

The `mItemOffsetY` is calculated incorrectly when ListView is set with divider as below:
```xml
<com.larswerkman.quickreturnlistview.QuickReturnListView
        android:id="@android:id/list"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:divider="#abcdef"
        android:dividerHeight="1dp" />
```
Here's a quick fix for the issue.