No illustration description for image on Background Priority settings screen (EXPOSUREAPP-4097) (#2031)

* added getter for illustration description in BackgroundPriorityState

* Added new string to other string files

* Removed unnecessary string entries

* Added actual string values.

Co-authored-by: Ralf Gehrer <ralfgehrer@users.noreply.github.com>
Co-authored-by: Matthias Urhahn <matthias.urhahn@sap.com>