Simplify service manager's tracking of singletons

It only tracks whether to dispose a singleton instance now, and (at least
conceptually) no longer remembers the single instance (apart from what is
necessary in order to call dispose on it), as the underlying implementation
already needs to keep track of that to support direct calls of constructor
functions.

Change-Id: I154bf05438e1db099c1c5ffb1c56377725c6bfc6