Update synonyms and fix some issues related to them

- update synonyms definitions
- fix issue with L release regression for indexing "Backup & reset":
was missing the marker interface "Indexable"
- fix non working synonyms for "Language & input": their declaration
was done in the XML preference file but was not used because the
SearchIndexProvider was built "by hand"

See bug #17474866 Settings search synonyms needs an update

Change-Id: Iceab71a0e3214f3a8ba0dcd77756c3632609c5ce