multibuttonentry: change key up callback into key down callback for consistency

multibuttonentry: change key up callback into key down callback for consistency

Summary:
Every elementary widget's action is triggered by key down event except multibuttonentry.
There's no reason why focus move and deletion should happen on key up in multibuttonentry.
This patch modifies _layout_key_up_cb into _layout_key_down_cb.

Test Plan: None

Reviewers: raster, Hermet, seoz, jpeg

Reviewed By: Hermet

Differential Revision: https://phab.enlightenment.org/D755