Make code compatible with php 7

When going to provide my provider settings in ad manager, under Slot-level customized targeting, my key value pair says array instead of the string value. This is cause because has chnage the way it interprets indirect expression. http://php.net/manual/en/migration70.incompatible.php

// Original expression
$entity->$entity_info['entity keys']['id'];
// PHP5: This is what we want.
$entity->{$entity_info['entity keys']['id']};
// PHP7: This is what we don't want.
($entity->$entity_info)['entity keys']['id']};