Allow specifying PYTHON executable to build_android (#41927)
Summary:
build_android.sh should check PYTHON environment variable before trying to use default python executable.
Even in that case, try to pick python3 over python2 when available.
Closes https://github.com/pytorch/pytorch/issues/41795
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41927
Reviewed By: seemethere
Differential Revision: D22696850
Pulled By: malfet
fbshipit-source-id: be236c2baf54a1cd111e55ee7743cdc93cb6b9d7
