layer: change logic for giving keyboard-interactivity

layer: change logic for giving keyboard-interactivity

Use the following logic:
 - Exclusive: Grant regardless of layer (previously it was only given if
   in top or overlay layers) AND grant if in the same or higher layer
   (nearer overlay) compared with other clients with exclusive
   interactivity.
 - On-demand: Grant only if no other layer-shell client has exclusive
   keyboard interactivity. Previously it was treated the same as
   exclusive.
 - None: Unset focus if the commit associated with the 'none' came from
   the currently focused layer. Previously it was just unset regardless.