Android: adaptive icon

Android 13 added support for [themed icons](https://developer.android.com/about/versions/13/features#themed-app-icons), and apps start to adopt it slowly. It would be nice if GitJournal had it too.

Themed icon require special monochrome layer in the [adaptive icon](https://developer.android.com/develop/ui/views/launch/icon_design_adaptive). Currently, GitJournal icon is not adaptive.

I can help with designing this monochrome icon and adding it to the app.