That is because Jolla uses systemd convention to mount auxiliary storage devices to "logged in" user's /run/user directory (in this case /run/user/100000) and that directory is protected from other user's on the system. Sailfish processes and apps run under nemo user (uid 100000) and Android apps run as another users (no usernames on /etc/passwd).

/run is "virtual" filesystem (tmpfs) so it doesn't survive booting. You can try to give other users access the nemo's /run/user directory but it needs to be done again after boot or put in the system's startup scripts. This might have some negative security effects.

More safe way might be this:

devel-su
mkdir /home/nemo/sdcard
mount -o bind /run/user/100000/media/sdcard /home/nemo/sdcard

After that SD card is available as /home/nemo/sdcard for all apps including Android. This needs also to be done again after booting. This seems to work. I tested it with FBReader's Android version.