Mark Layer as modified when changing zOrderRelativeOf.

If the layer is not marked as modified, the changes will not get copied
from current to drawing. This will break drawing state since it will be
missing info about what a layer is currently relativeOf.

Fixes: 125468401
Test: Can no longer repro issue in bug.
Change-Id: Id75a2bc87c179c4bd11141032acc612e08f0db7b