Include createdTimestamp and createdDate in pings

[st3fan]

Frank said on Slack:

okay, I’m going to make a suggestion, which is going to be part of all mobile telemetry moving forward, and will then be standardized across all mobile pings
createdTimestamp, an integer
and createdDate, a string, of YYYY-MM-DD
then whatever all these different pings do with created can eventually just be removed

createdTimestamp is a unix timestamp in milliseconds (millis since epoch)