I learned to sight read music 20 years ago but needed a huge refresher. However, I only learned the treble clef back in the day, not the bass clef, and the piano uses both.
Skoove doesn't advertise that it has an Android app version, but I found it by accident, and find this program most useful when I use the app and the online version in conjunction with one another. The app shows some of the finger numbers above or below the notes on the stave, which is really useful to get used to which key correlates to which note, whereas the online version doesn't, so you're trying to learn the keys on the piano and to sight read music at the same time instead of being helped out with one thing so you can concentrate on the other.
It has a music theory lesson, but again, it only deals with 5 notes on the treble clef stave and doesn't bother with the bass clef, so it's not overly helpful.
I'm a few weeks in, and I'm getting on quite well, however, I am not a beginner musician having played the guitar for 30 years, so I have some degree of musical ability and knowledge. I would have to say that if I didn't use the app in conjunction with the online version I probably would have lost patience with it by now because it expects you to sight read music from the get go.
I don't know that I will bother renewing and paying for a further subscription once the 6 months is over, but it's been somewhat useful to get me into it.