AND-116 Fix onboarding animation edges

[Radiokot]

Purpose

If the screen is not too tall, the animation doesn't reach left and right edges. It is not as visible on a screenshot, but the cutoff is obvious in motion.

Changes

• Make the confetti fit the screen and fade

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.