Adaptive icons for Android Oreo&up

Currently building of icons is handled [here](https://github.com/reflex-frp/reflex-platform/blob/develop/android/buildIcons.nix), and it only supports raster icons. Modern Android (Oreo and up) instead makes use of [adaptive icons](https://developer.android.com/guide/practices/ui_guidelines/icon_design_adaptive). We should support creating adaptive icon resources based on SVGs.