setSafe

Set the Factory that will be used to lazily create the singleton ImageLoader.

This function is similar to setUnsafe except:

• If an ImageLoader has already been created it will not be replaced with factory.

• If the singleton ImageLoader has already been created, an error will be thrown as it indicates setSafe is being called too late and after get has already been called.

• It's safe to call setSafe multiple times.

The factory is guaranteed to be invoked at most once.