The "Background" metrics category was being set in the
`backgroundMetaMetricsEvent` function. This function might not
necessarily include any event at all though, so setting it here seemed
inappropriate. It would also crash if `eventData.eventOpts` was not
set, which is not great since that property is optional.
The background category is now set in the `sendBackgroundMetaMetrics`
function in `metamask-controller`. This method is used solely for event
data, so it would make sense for this category to be always set.
There is no functional difference, since `backgroundMetaMetricsEvent`
is called solely by `sendBackgroundMetaMetrics`.
