+1 on the dammp chaser. I installed one in my piano in January and then tuned it in February and it's been remarkably stable. Normally the humidity swing between dry February and humid May throws the piano out, but I haven't noticed it this year.
On the installation, it was recommended to drill a hole through the bottom of the piano for the electrical cord or to route it out the top or above the base board and around the side. I didn't like those options so I went through the corner of the soundboard, which isn't acoustically active.

Re: tuning apps…If you don't mind my biased recommendation, I think that for tuning your own piano with Android, the free/trial of Tunelab or "plus version" of PianoMeter are the most straightforward+affordable way of doing it. I've seen some people tune using the "free" version of PianoMeter as well, just matching up the blue dots to the tuning curve. (That seems tedious to me, and I don't exactly recommend that.) Also, if you don't mind a little tedium, there is also the Entropy app, which is free if you can find it. (I think the main website is down now, and I couldn't find it on the app/play stores last time I checked, but there are probably some APK files still floating around the internet.)