status/keyboard: Adapt to external source of keyboard layout

status/keyboard: Adapt to external source of keyboard layout

This adapts to API changes that allow an external source (e.g. remote
desktop API user) to control the current keyboard layout. This is done
in two ways:

If the external keyboard source sets the keymap as "locked", the
indicator is fixed to the short name of the external source, and will
change the indicator label when the layout index is changed accordingly.
The builtin input sources are not selectable, and the keyboard selection
keybinding is disabled.

If the external keyboard source doesn't set the keymap as "locked",
whenever the current keymap is from an external source, it's indicated
as a non-sensitive but selected input source. Selecting another keyboard
layout overrides the one selected by the external entity.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3901>