efl_ui/layout: fix multiple emissions of theme,changed during construction

efl_ui/layout: fix multiple emissions of theme,changed during construction

in the case where a layout object was created and had a theme manually set
with efl_ui_layout_theme_set() during construction, the layout would then
call theme_apply() a second time internally during finalize which, if the
theme has not changed (as can only be the case if this flag is unset),
results in a repeated theme_apply for the existing theme

@fix

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D10738