This app was mentioned in 3 comments, with an average of 1.67 upvotes
I just use it the same as an incident light meter, holding it up to my subject to grab the amount of light. If you need help using a light meter let me know because it took me a while to get it when I first started.
This app has always worked well and given me perfect exposures. https://play.google.com/store/apps/details?id=jp.gr.java_conf.appdev.app.exposuremeter
I use this one on my moto x-play:
https://play.google.com/store/apps/details?id=jp.gr.java_conf.appdev.app.exposuremeter
It's bang on and seems to work great.
I tried a couple before I found one that seems to be accurate enough to actually use.
Android I use Light Metre For Photo