Add a toggle widget for the FTP server, that allows the server to be switched on and off directly from the home screen. Make the icon reflect the status.

Bonus: include an option 'turn server off x min after last activity'
Bonus: include an option 'turn server on when connected to WLAN xyz'

Examples from the competition:
https://play.google.com/store/apps/details?id=org.primftpd
https://play.google.com/store/apps/details?id=net.xnano.android.ftpserver