GPII-778: Adding "deviceReporters" properties.

Added "gnome-shell" as the app to look for with respect to
GNOME Shell overrides (org.gnome.shell.overrides).

Similarly, changed GNOME Assistive Technology - Screen Keyboard
(org.gnome.desktop.a11y.applications.onscreen-keyboard) to
"gnome-shell" (was "gsettings-desktop-schemas").