Performance optimizing actor-impl & relayout controller

Performance optimizing actor-impl & relayout controller

- Store child actors as intrusive pointers rather than handles
- Stop using public handles in internal code in actor (removes about a dozen unnecessary GetImplementation calls)
- Stop calling Stage::GetCurrent().GetSize every size negotiation, store it internally instead

Change-Id: I18615aa1f8d2c736cd8502fa02de55e9146a1ef6