send user_image only on public lti launches

test plan:
  - change a tool's privacy level to anonymous
  - tool launch should not have user_image
  - change a tool's privacy level to public
  - tool launch should have user_image

fixes PLAT-708

Change-Id: Iccca705fd3e5c5385ae63897e49d9c67d46f62d7
Reviewed-on: https://gerrit.instructure.com/43633
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
QA-Review: August Thornton <august@instructure.com>