Beginner

Gradle, Manifest, Widgets

Toolbar, Actionbar

Livedata, Data Binding

Webview, Fonts, Richtext

Internationalization

Intermediate

Data Persistence (SharedPreferences, SQLite, Room)

Concurrency (Coroutines, Threads, WorkManager)

Networking (API, JSON, XML, Authentication, Authorization)

Architecture (MVI, MVP, MVVM)

Dependency Injection

3rd Party Libraries

Usually

Gson (https://github.com/google/gson)
Koin (https://insert-koin.io). Glide (https://bumptech.github.io/glide)
OkHttp (https://github.com/square/okttp)
Fabric (https://firebase.google.com/docs/analytics)
Retrofit (https://square.github.io/retrofit)
Realm (https://realm.io)

Sometimes

Chuck (https://github.com/jgilfelt/chuck) Stetho (https://facebook.github.com/stetho) Autodispose (https://github.com/uber/AutoDispose) RxBinding (https://github.com/JakeWharton/RxBinding) gRPC (https://grpc.io/docs/tutorials/basic/android)
Mockito (https://github.com/nhaarman/mockito-kotlin)
PowerMock (https://github.com/powermock/powermock)
Facebook Login (https://developers.facebook.com/docs/facebook-login/android)
Google Login (https://developers.google.com/identity/sign-in/android/start-integrating)
OkHttp Interceptor (https://github.com/square/okhttp/tree/master/okhttp-logging-interceptor)
RootBeer (https://github.com/scottyab/rootbeer)
Zxing (maintenance mode, replaced with Firebase: https://firebase.google.com/docs/ml-kit/android/read-barcodes)
Lottie (https://github.com/airbnb/lottie-android)
Zetbaitsu (https://github.com/zetbaitsu/compressor)
Jackson (https://github.com/FasterXML/jackson-core)
Shimmer (https://facebook.github.io/shimmer-android)