send user id for ContentItemSelectionResponse launches

test plan:
 - Install a tool with a message type ContentItemSelectionResponse
 - it should contain a opaque user_id

Change-Id: Ic2f6585adfe2d9792f974d1d58db2b82da2cdff2
Reviewed-on: https://gerrit.instructure.com/40718
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>