collation: Use `self` to reference same class instead of class name

collation: Use `self` to reference same class instead of class name

With the use of `self`, even if the class name is changed (which is
very unlikely), then the code won't break. So it's a way of future
proofing the code.

Change-Id: Ib4046315b8aaee2e77af036893924962d25ceeb1