Code style improvements (#346)

* Code style improvements

- Use final modifier to express values which should not be modified
- Use @NonNull / @Nullable annotations for more semantic
- Consequently deprecate V1 APIs (#133)

Signed-off-by: Stefan Niedermann <info@niedermann.it>

* Drone: update Lint results to reflect reduced error/warning count [skip ci]

Signed-off-by: drone <noreply@drone>

Co-authored-by: drone <noreply@drone>