The CONSTRUCTED state was only briefly set between constructSingleton() and
finishInitializing(). But as no consumer code is executed between setting
CONSTRUCTED and setting INITIALIZING, it was impossible to reach the switch
statement in either getInstance() method in state CONSTRUCTED. So there was no
point in state CONSTRUCTED. Remove it.

With CONSTRUCTED gone, we only ever call finishInitializing() right after
constructSingleton(). Merge finishInitializing() into constructSingleton().