Commit 188811f808 ("scene_graph: Port layer_shell") accidentally
removed code in `arrange_layers` to handle focus on layer shell
surfaces with keyboard interactivity.
Due to this, layer shell surfaces requesting exclusive keyboard
interactivity may not get automatically focused, and layer shell
surfaces giving up exclusive keyboard interactivity can remain focused.
Add the previous code back to fix the problem.
Note the non-rename change included in b4d7e84d38 ("desktop: Rename
layers to shell_layers") is not included as it also seems accidental.
Fixes: #7936
4d4c88f0a7