Mobile events, updates to video events

@stroilova , @mulby , @nasthagiri , @gwprice , @lou-wang , @mhoeber here is a framework that we can work from to get initial documentation out with the release. Notes:
- I am not sure that the same additional context fields that I see for the v.0 play_video (Olga shared with me) event also apply to all of the other video events.
- the event field names for the new edx.app.browser.launched event in Olga's spreadsheet are very rough
@nasthagiri  It would be good if the current_time was emitted in the same format throughout all events, and that that was consistent. I did see some discrepancy between different event types. The video player in the browser currently emits sometimes floats, sometimes decimals to 1 decimal point, and sometimes integers. Could we standardize this in the mobile app? I am not sure what the technical reasons are for this disparity...

@pmitros may I ask -- do you have any insight here on the format that would be best for storing current time in video events for research, or the history between different number formats? We are hoping to standardize, if possible...