Add 'yyyy' to nomalizedDate method map for adminhtml i18n

[Amakata]

• Sample Code

<?php


$t = (new \IntlDateFormatter(
   'ja_JP',
   \IntlDateFormatter::MEDIUM,
   \IntlDateFormatter::NONE
))->getPattern();

echo $t, PHP_EOL;

Mac homebrew php56 + php56-intl result: y/MM/dd
But, Amazon Linux 2015.09: php56 + php56-intl result: yyyy/MM/dd

Adminhtml datetime Correct: "2016/02/28 20:21:59"
Adminhtml datetime Result: "2016yyy/02/28 20:21:59"

in Japanese Locale Admin User on Amazon Linux.

[piotrekkaminski]

Internal issue MAGETWO-54369