my_activity.py tries to determine the current user with the USER
environment variable but on Windows it should use USERNAME. This change
gets it to check both, which saves users from having to use the -u
option and figure out what format the name should be in.

Change-Id: Id2458d29f8a2635a96dd3859d6af0ed5aaea22bf
Reviewed-on: https://chromium-review.googlesource.com/c/1479896Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>