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

python-packages/adb/setup.py

@@ -24,6 +24,7 @@ setup(
     keywords='adb android',
     package_dir={'adb': ''},
     packages=['adb'],
+    package_data={'adb': ['py.typed']},
     classifiers=[
         'Development Status :: 2 - Pre-Alpha',
         'Intended Audience :: Developers',