The ttf files must be referenced somewhere in books CSS file(s) using
@font-face rule and/or you have to know where does your preferred epub reader Android app expect to find extra/custom fonts, then copy the files there.
Or to make things simple you could embed the font files (using Calibre, first you have to add/install "STKai" font on you Windows or add them just to Calibre) epub itself, for that 1 book. I'm guessing that you Android app support embedded fonts...