I'm trying to get to N4 level in JLPT. Here are two apps I use on my Samsung phone i.e. 'Beginner Japanese' and 'JLPT Locker' apps.
Both apps are free and most importantly, works offline.
The former, Beginner Japanese, is a word a day concept, except, they allow you to learn up to 12 a day or more depending on your answers (yes, they quiz you periodically on the words you 'learned').
JLPT Locker is a bit for more advance, as they assume you know all the words in the level (e.g. N5, N4 etc) and test you on all of it in the level.
Btw, if you don't already know, JLPT stands for Japanese Language Proficiency Test, and its the default goto language exam recognized by the Japanese government. Both apps incorporate the common words used in the JLPT exams.
https://play.google.com/store/apps/details?id=com.shex.beginnerjapanese https://play.google.com/store/apps/details?id=com.shex.jlptlocker