You should not worry about have your data as arff
file, since Weka supports other input formats. You can simply have your data as csv
file and imported to Weka. You can export it from Weka as arff
if you want.
There is a port for Weka to android that you can download here, I did use it in my project without a problem. You can train your model in the PC and export it, then use it in android see my answer here for example code.
Yes, Weka supports Stemmer algorithms