Add py.typed file to export types to dependents.

There isn't any type info yet, but this is a prereq for exporting that
data.

https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-library-stubs-or-py-typed-marker

Bug: None
Test: None
Change-Id: I9fa7a540a57ff76a4f8dcded92456b8c4be09dad
2 files changed