Check if newly focused slug is different to previous slug

The old logic only checked if we went from inSlug to !inSlug (or
vice-versa). The missed the case when we jump from one slug to another,
so instead change focus classes if the new slug is different to the last
one.

Bug: 62783
Change-Id: Ia2f33c4d4ce37d58a9661e3a837824f10ba6804b