Build FFmpeg-kit ourselves to reduce apk size

See https://github.com/tanersener/ffmpeg-kit/wiki/Building-Android and https://github.com/tanersener/ffmpeg-kit/discussions/423#discussioncomment-2765934

Currently the apk is around 77Mb mainly because of ffmpeg (and that's with the ffmpeg-kit-min package). Ouch. We don't need most features it provides so if we build it we could save a lot of space.