Fix #1426: App crash at launch on iOS 12 devices.

[iccub]

Summary of Changes

For some reason the app was crashing only on iOS 12 devices. To fix that I would have to override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?)

but then the stored property onboardingType was always nil even when I assigned it.

So I changed the implementation to use a convenience initializer and now it works for both iOS 12 and 13

This pull request fixes issue #1426

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

Launch the app on both iOS 12 and 13, and see if it crashes.

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).