Revert "Temporarily disable `arcs_kt_plan` invocation" (#4931)
Revert "Temporarily disable `arcs_kt_plan` invocation" (#4931)

* Revert "Temporarily disable `arcs_kt_plan` invocation (#4930)"

This reverts commit 8e20d4168def391107f5fd286027decdbff6f62c.

* Rename core Handle classes (#4917)

* Rename core Handle classes

`SingletonImpl` -> `SingletonHandle`
`CollectionImpl` -> `CollectionHandle`

Also, all of the `SetFoo` typealiases have been renamed to `CollectionFoo`

* One more fix

Co-authored-by: Cameron Silvestrini <a94e2024bc38369a2fd9d1d423706db94a486341@users.noreply.github.com>
Co-authored-by: Alex Rosengarten <7f53ebb9ae575dfae4c3877b459464b8c65a2dfc@google.com>
PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs