Common: consider mapping GA adapter trackingId to app_id

[chuwy]

It's getting requested by multitenant users https://github.com/snowplow-incubator/snowplow-google-analytics-plugin/issues/23, https://discourse.snowplowanalytics.com/t/snowplow-google-analytics-plugin-does-not-send-app-id/2224

[jeroenvlek]

In favor of this issue, as it seems like a small fix and prevents empty app_ids in the atomic table.