Move the `mGens` array and `isAlive()` implementation from `EntityManager`
to `EntityManagerImpl` to hide implementation details and maintain a clean
public header. Make `isAlive()` strictly thread-safe by acquiring the
`mFreeListLock`.
isAlive() was relying on undefined behavior, and the comment about
the memory barrier being provided by the lock was misleading (wrong)
because the lock only provided the "release", but isAlive() didn't
have an "acquire".
Better safe than sorry, we now have to acquire the lock to test isAlive().
547f1f7e32