Fix a bug that ExpandableGroup#getPosition returns wrong index

[CORDEA]

Currently, ExpandableGroup#getPosition returns the same value (0) for parent item and nonexistent item.

@Test
public void testGetHeaderPosition() throws Exception {
    ExpandableGroup expandableGroup = new ExpandableGroup(parent);
    assertEquals(0, expandableGroup.getPosition(parent)); // parent position
    Section notAddedSection = new Section();
    assertEquals(0, expandableGroup.getPosition(notAddedSection)); // nonexistent item position
}

But Section and UpdatingGroup seems to return -1 when target group does not exist, so I want to make ExpandableGroup the same behavior. This makes it clear that 0 is a parent item and -1 is a nonexistent item.

Please try to consider this ;)

[lisawray]

Seems reasonable!