Re-parent invoked on child instead of on parent.

The function to re-parent an individual child is now invoked on
the child instead of the parent. This ensures the child ends up with
the last parent set if multiple reparent requests are made in the same
transaction.
This also allows adding a parent to a layer that didn't have one
previously.

Test: Transaction_test -> Reparent, ReparentToNoParent,
ReparentFromNoParent

Change-Id: Idab429eb2dca5a4ae1b020a5a7629d719dd4d995