rename BUILD_ATEN_MOBILE to INTERN_BUILD_MOBILE and make it private (#19942)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19942
ghimport-source-id: 6bacc8f5ad7911af8cf5fde9fcb604ade666b862
Reviewed By: dzhulgakov
Differential Revision: D15144325
Pulled By: ljk53
fbshipit-source-id: d63a70f007110d5d1055d6bec1ed09a1a6aafdae
