Fix rebuild with cells

When rebuild_instances switched to a compute task it broke
cells since the cast was dropped as it didn't match.

This change moves the rebuild_instances to conductor and
also adds some unit tests checks on the compute
task integration point.

Change-Id: Ic1c575fbca72f1cd34b1dbc6f82023e5b2b392e2
Closes-Bug: 1348642
Merge "Fix rebuild with cells"