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 There will also be many events with types other than "track".  In particular, all "screen" and "identity" event types are ignored and are listed in the logs.