Improve difficulty icons

android/gradle.properties

assets/icons/difficulty_easy.png

assets/icons/difficulty_hard.png

assets/icons/difficulty_medium.png

assets/icons/difficulty_nightmare.png

fastlane/metadata/android/en-US/changelogs/16.txt

fastlane/metadata/android/fr-FR/changelogs/16.txt

icons/difficulty_easy.svg

icons/difficulty_hard.svg

icons/difficulty_medium.svg

icons/difficulty_nightmare.svg