[ticket/12170] Assign schema keys in migration helper with data_depth 1

The migration helper currently drops any keys with schema changes that have
the data_depth 1. This change will correctly assign the keys to the steps
array without dropping the keys that might contain important info like the
actual table that should be created.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-12170

PHPBB3-12170