Add indexing for ListPreferences

- now support ListPreferences and save the "entries" attribute
- update Index database schema (and increment its version)
- do some clever stuff when showing Search results: if there is
a "$s" or "%s" in the summary (replacement strings), just use
the entries instead

Change-Id: If36595c3816706b6349faff7d3c2e725d3ea33f4