VPN-6681 part 2: iOS and Android launch screens, Debug menu to show all images

[mcleinman]

Description

Finishing touches for asset-related dark mode work:

• Android and iOS now have launch screens that respect the system's color scheme. Important: This follows the system's dark mode/light mode, not our app's theming. (We cannot follow app theming, as launch screen is setup happens before app is fully open. Additionally, following the system's color mode preference makes sense for the launch screen, which is seen in the transition from the system to the app.) One consequence of this: This goes live immediately, and isn't behind a feature flag.
• The new debug option for an image playground now works - it shows a chosen image 4 times - against both light and dark background, and both backgrounds have the light and dark version of the image.
• The animation playground has been updated so the user can select one of several colors to use as the background.

Reference

VPN-6681

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed