Update printers even if there are no printers found.

It can happen that before we stopped we reported a set of printers. Then
while the loader is stopped, we lost all printers. Then on re-start we
need to tell the clients that there are no printers anymore.

There is one issue: The fused-printers-provider might be started
before the printer registry is fully created. In this case we get a callback
and get all king of NPEs. Hence enqueue the first load behind the
current thread. This makes sure that the current operation - creation of
the printer registry - is finished.

Bug: 27643300
Change-Id: Ife6993d57457f23a0a4a54a11869c742c05da850