MA-157 Move context back into properties.

https://openedx.atlassian.net/browse/MA-157

Here's the updated gist from android-generated video events:
https://gist.github.com/nasthagiri/58ec88c1a92e6641dd0e

ARCHIVE (before BI blacklist and before android-app updates):
Here's a gist of the events in the event log: https://gist.github.com/nasthagiri/8d9e83734791cfc4950e
@mulby As discussed, we'll keep this as an error message for now, in case it will be helpful to debug any webhook issues on prod.  We will remove this error message a few weeks after enabling on prod.