custom ringtone support: Add Custom Ringtone for incoming calls

This allows to add a custom ringtone per contact. Either system one or imported sound.

Test: sudo ubports-qa install focal_-_custom_ringtone ( reboot needed and probably sudo umount /usr/share/session-migration/scripts before...) To set a custom sound go to contact app, edit a contact, select “Ringtone" field, set a sound ( you can import a custom one). Don't forget to save the contact. An incoming call from that contact should play the sound