It was much quicker than I thought.
But apparently, "audio latency" is one of the Android's unsolved problems.
It's... kinda not very simple to keep a steady tempo on the real devices, especially older ones.
I adjusted settings and programming routines according to my test devices' behaviors though.
Hopefully it works on other devices as well.
Anyway, I'm sure this tool is very useful for the guitar learners who want to practice "Arpeggio" play!
Please let us know if you experience any sound problems.