Fix two problems with export and alphabetical headers

* LT-13819 - the previous code to map secondary and
  tertiary sort rules to the main character was wrong
* LT-14260 - there was no attempt to handle the mapping
  of digraphs and secondary and tertiary characters when
  the sort rules were 'simple'

Change-Id: Ib9bc8e8a72398ac1a85062ae6f23a2278184e9ed