Move storage/louds/key_expansion_table{.h, _test.cc} files to dictionary/system

Key expansion table is no longer associated with louds trie but system dictionary.  So it should be located under dictionary/system.

This is just a code refactoring, no user-visible behavior change is intended.

BUG=none
TEST=unittest

git-svn-id: https://mozc.googlecode.com/svn/trunk@527 a6090854-d499-a067-5803-1114d4e51264