Add anydpi icon, plus simplified splash screens.

[freakboy3742]

A big simplification of the icon and splash screen configuration for Android.

• Adds an "adaptive" image variant. This appears to be necessary on recent versions (> API 25?) to get the icon on the launcher to render without the extra whitespace around the icon. The splash_background_color field has been adapted for use as the background of this icon.
• Removes some unused launcher resources (drawable-v24/ic_launcher_foreground, drawable/ic_launcher_background) that appear to be unused leftovers from a default template.
• Replaced the drawable-*/launch.png "made with beeware" image with a mipmap-*/splash.png image that is the same as the icon
• Simplified the splash screen definition so that it doesn't need as many image variants.

The explicit splash screen is only used on very old Android versions (possibly only v7 and v7.1); more recent Androids use the adaptive icon variant. This splash screen is essentially just the app icon, centered on the launch page.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct