Remove from top layer list if re-parenting top layer.

If a layer is given a parent when it originally didn't have one,
remove the layer from the top layer list. This is to ensure the
layer isn't in duplicate places and that it can get properly
cleaned up.

Test: Transaction_test#ReparentFromNoParent successfully removes
layer when test completes.

Change-Id: I1f1c4ee386626c1ef2e3a2e50324ec336d234d5b