Constructor functions for singletons still need to pass out single instances

...as they are not only called from the service manager (which takes care of
singleton constructor functions since 997d21183322a0a94b96868073808841d2773902
"Support for singleton constructor functions") but potentially also directly
from cppumaker-generated code (which is the raison d'être for constructor
functions, after all).

However, this change:
* postpones the instance's destruction to atexit, with all dreaded consequences;
  lets see how that pans out.
* makes it questionable whether the service manager holding references of these
  singletons (introduced in 997d21183322a0a94b96868073808841d2773902) is
  necessary after all; lets revisit that in another commit.

Change-Id: I0a2c902ab62045df1af382d190747e34b80236d3