Expose direct access to MockExternalizer object used by AemContextImpl

The MockExternalizer contains the method setMapping(...) which is supposed to be used to set a custom mapping.
Unfortunately the MockExternalizer object used by AemContextImpl is not exposed directly but only as Externalizer service. It would be nice to expose the MockExternalizer directly from AemContextImpl, as currently one needs to get the Externalizer service and then cast to a MockExternalizer to be able to call its setMapping(...).