Move special cells logic for start/stop

This moves towards better integration of cells logic by removing
the special start/stop code in cells_api.ComputeCellsAPI().  Instead,
the behavior difference for cells/non-cells for start/stop will only
be because of the different compute RPCAPI class.

Change-Id: Ie9bf694351d5f7f49c60b7d1ab5c4b2a679e3ab0