PresetStore::open built its path from XDG_CONFIG_HOME or HOME. Android
sets neither, so the library resolved to /.config/darkroom, which is
read-only, and every preset saved on the tablet failed. Windows sets no
HOME either and got a directory relative to the working directory. The
settings store was moved to dr_sync::account::config_dir for the same
reason in 0.12.1; the presets now follow it. Linux and macOS resolve to
the same file as before.