status/keyboard: Ensure we always have at least one input source

Most of the code handles the sources setting being empty and
InputSourceManager.currentSource being null because previously the
"model" (i.e. the sources list) was kept in gnome-settings-daemon.

But this is fragile and since we're now the canonical place where the
list lives we can force it to never be empty even if the gsetting is
empty or contains only invalid entries.  Adding the default keyboard
layout in that case is the safest thing to do.

https://bugzilla.gnome.org/show_bug.cgi?id=738303