Fix item count check in Folder#replaceFolderWithFinalItem()

Instead of checking the item count (number of views), check the number
of items in the model, since that is where the item is being removed.
This way, mInfo.contents.remove(0) will never be out of bounds.

Bug: 25666490
Change-Id: I0eb880b366157bddb5a8c342d538737722b7f0cd