Incorrect timestamp format in AndroidNotification.event_time #337

I think the way we format the event_time field on AndroidNotification is wrong:

There seems to be couple of issues with this.

1. We are not specifying the timezone for the SimpleDateFormat. Therefore the resulting timestamp string is in the local timezone while the backend expects it to be in UTC.
2. The way nanoseconds are formatted seems to be wrong. For the milliseconds value 1546304523123L this generates the string 2019-01-01T01:02:03.000000123Z. I would expect the nanos to be formatted as 123000000 (i.e. 123 milliseconds in nanos).