[elm_index] changed priority rule

[elm_index] changed priority rule

according to priority, the index shows only one selected group at a time
and the other groups are represented by one item.

before:	always the first item of a group represents the group
after:	if the priority of a group is lower than that of the showing group,
	the last item represents the group

Change-Id: Ie16839526229b8687f5752e2f798cdb992c31a73