Concurrent access to WorkEventBus#getOrCreateFragmentManager leaks memory.

WorkEventBus uses a ConcurrentHashMap to ensure there is one and only one FragmentManager corresponding to each unique FragmentHandle. The method creates a FragmentManager that reserves some initial memory if a thread observes that it is not in the map. However, in case of concurrent access, we still need to clean up after if multiple threads observe that FragmentManager does not exist and create multiple FragmentManagers.