Code logic will never reset password expiry

If the password is expired and the user tries to reset his password, the password will be reset successfully but the flag for expired password will not be removed due to undefined variables in the code.

$uid = $form_state->getValue('uid');

$form_state->getValue('uid'); is undefined since the uid in Drupal 8 isn't part of the form state object.

This should be replaced by:

// get the uid from user object.
$user = $form_state->getFormObject()->getEntity();
$uid = $user->id();