Add ApiModuleManager::getClassName()

In cases where an extension has multiple API modules that
have the same exact factory requirements, it would let them
use the same factory function and check with the module manager
what the class name to construct should be. This is already possible
using getNamesWithClasses, but this makes it much more
straightforward.

Change-Id: I889e3e6f15907896a8df376913125d903debe984
Add @covers tags to ApiModuleManagerTest

Change-Id: Iaac773576a663d2f426632ad95f94b3556ef15b2