kiosk-shell: Update view transform after activation.

The activation of a view implies, among other things, a change in the
associated view layer which is initially unset. In order for this change
to be reflected in the corresponding surface's output mask, and hence
allow surface damage to trigger output repaints, we need to update the
view transform.

Fixes #674

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>