There is no magical Android app that formats your paper while you nap. Formatting is still LaTeX on a real computer. What an app can do is catch the dumb mistakes, and the dumb mistakes are the killers.
- Template support: does it handle the venues you actually submit to? - Reference checking: mangled citations cause more desk rejects than bad results - PDF validation: margins, fonts, page limits, the holy trinity of rejection triggers - Free covers the basics; pay only if you submit to the same venue family every year - Write on the real machine, validate with a checker, track dates on the phone
For the final pass, VenueFit is free and the formatting service gets your paper submission-ready for the venue you picked.