Add annotations to methods and parameters.
Replace Jetbrains' @NotNull with Androidx' @NonNull annotatiation.
Make class variables static if possible.
Use constants for some Strings.
Simplify if conditions.
- Introduce Groupie for easier lists implementations
- Use some of the new components of the Android Architecture libraries
- Add a bunch of icons for groups, using vectors, which still is
compatible with older APIs through the compatibility layer