fix #2317: NPE while backuping data

The previous fix for #2317 was incomplete, as the dependency of the
CentralBackupAgent onto Settings caused a NPE because Settings static
initializations require that an application has been created, which is
apparently not the case when a backup is launched by the OS.

The preference name has been isolated into a no-dependency package so
that the CentralBackupAgent does not depend on an application existing
anymore.