@Px abstract fun itemInsetBefore(groupCount: Int, groupIndex: Int): Int
Defines a custom inset that will be applied on the top of each element for a vertical list and on the left of each element for an horizontal list.
groupCount - number of groups in a list (equal to the list size when the span count is 1).
groupIndex - position of the group (equal to the item position when the span count is 1).
Return
inset before the item (expressed in pixels).