Make displayChange in existing transition just an error.

This doesn't need to crash the system. It should be an error,
though. Unfortunately, display-changes, currently are not
deferrable so we can't guarantee a unique transition.

Bug: 259004137
Test: systemui-foldable-smoke-suite
Change-Id: Iea946724fa949d731f05f1f975ffd8b338f78684