Fix bug where app launch messages weren't appearing on iOS

[christibbs]

There were two issues:

1. App launch FIAMs come through with a fiamTrigger param in the payload (as opposed to the event param for general analytics events) and we were only validating messages with a fiamTrigger of ON_FOREGROUND.
2. We needed to add a flow to check for app launch FIAMs at the appropriate time-- after the initial app launch fetch occurs and messages are retrieved and stored into the cache.