efl_ui_timepicker: change a property name (ampm -> is_24hour)

Summary:
For better understanding of the property, "is_24hour" looks
better than "ampm".

@ref T7901

Reviewers: zmike, segfaultxavi, bu5hm4n, kimcinoo

Reviewed By: segfaultxavi, kimcinoo

Subscribers: kimcinoo, cedric, reviewers, committers

Tags: efl

Maniphest Tasks: T7901

Differential Revision: https://phab.enlightenment.org/D9811