UTM parameters lost after relaunch

### [REQUIRED] Step 2: Describe your environment

* Xcode version: 10.2.1
* Firebase SDK version: 5.19.0
* Firebase Component: Analytics
* Component version: 5.8.0

### [REQUIRED] Step 3: Describe the problem

We’re having a problem with utm parameters. We track those with `AnalyticsEventCampaignDetails` event whenever a link with with utm params are clicked (we're using branch.io for links) but we’ve seen that conversion events are not attributed to utm params. More specific, _utm params are lost after a relaunch and future events are not attributed_.

Facebook persists utm events and attributes future events with utm params. Firebase should also do the same, per spec (https://support.google.com/firebase/answer/6317518?hl=en) In the meantime, we've implemented our custom interim solution: save utm params to user defaults and report them on relaunch.

We’re searching for a way to test this functionality. Specifically, we want to make sure our utm parameters, once reported, gets attributed to future conversion events, so we don't need our custon solution. DebugView does not show utm params on events. How should we proceed?

Also asked the question under issue https://github.com/firebase/firebase-ios-sdk/issues/2462 and told to create a ticket for it. Asked the question on firebase slack but could not get a response.