Add screenshots to F-Droid

Screenshots will help users evaluate the appearance and functionality of the `TimerDroid` before installing it.

[All About Descriptions, Graphics, and Screenshots](https://f-droid.org/en/docs/All_About_Descriptions_Graphics_and_Screenshots/)
[Fastlane file structure / Quick Fastlane Cheat-Sheet](https://gitlab.com/snippets/1895688)