media-keys: Make power-actions non-interactive based on mode

gnome-shell does not allow to grab any particular key combo more
than once, even when the corresponding keybinding modes don't
overlap, so the FOO_KEY_NO_DIALOG shortcuts introduced in commit
b7827c13 never actually worked.
Instead, use the new 'action-mode' parameter that was added to the
'Activated' signal to determine whether to a power action should
be interactive or not.

https://bugzilla.gnome.org/show_bug.cgi?id=741762