Add adaptive icon

[Commenter25]

Someone tried to make an issue about this with #245, but they deleted their account and didn't elaborate well on the issue.

Most apps use an icon marked with a safe zone that can be cropped to different shapes for different styles. Survival Manual does not do this, which makes it inconsistent with the rest of the system. Fixing this is a minor thing, but a major aesthetic improvement. Notice in my picture below that, besides not fitting the desired shape, the icon is actually subtly bigger than the others.

https://developer.android.com/develop/ui/views/launch/icon_design_adaptive

[dennisguse]

This also requires to add the icon as vector graphic (not yet in the repo).
It was done by them: https://cookicons.co/