Fix/onboarding analytics

[cnguyen812]

This PR refactors some of the onboarding logic so that if resuming onboarding from a closed app, analytics is still initialized.

• refactored the route caching to be a little more robust: properly memoized the debounce and used the index prop instead of length - 1 to read the child route properly
• previously it would always cache the Onboarding Notifications screen
• previously, analytics was only init from Onboarding Start, if the user closed and reopened the app it wouldn't kick off analytics
• added a queue for Segment events so any events that happened to fire before analytics has a chance to init get queued up
• removed our call to init the AppsFlyer SDK because the Segment SDK AppsFlyer plugin we use does this internally (we still require the AppsFlyer dependency even though it's included in the plugin because of pods and stuff)

Smaller stuff:

• moved some side-effects out of the app reducer function
• removed some unnecessarily nested promises (anti-pattern)