LTI: add XAPI duration to enrollment activity time

Change-Id: I2e381a0a65b81d8f1ad31562b67caf74d5962f84
Reviewed-on: https://gerrit.instructure.com/37116
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>