Refactor singleton type registration code

Refactor singleton type registration code

Previously each singleton type was registered as an implicit separate
import.  This commit changes the code so that these types are treated
just like any other type in the registration sense.

It also ensures that singleton types are instantiated per-engine.

Change-Id: I5c81c4ca5bf65210f7125d74a62a282a21838068
Reviewed-by: Matthew Vogt <matthew.vogt@nokia.com>