Centralize management of coordinates ready on GPU event

Centralize management of coordinates ready on GPU event

This change shifts the ownership of the EventSynchronizer
indicating that coordinates are available on the device from
UpdateConstraints to StatePropagatorDataGpu.
The latter now provides a getter that, based on workload flags, decides
which synchronizer to return. If update is offloaded the returned event
records the completion of the update-constraints on the GPU.
When update is not offloaded and on search steps the returned event
records the completion of the host to device coordinate transfer.

Change-Id: Idec9f46e78a2708fa31e895bf7590cdad1872987