Store raw strings for personal dictionary

The raw strings would be sent to personal LM for decoding.
Earlier lowercased strings were being used with the purpose
of isValid checks (spelling does not consider casing for spell
checking calls). But for showing these in suggestion, we need the
raw strings.

Note: PersonalDictionaryLookup#getWordsForLocale is used to feed
the personal LM in PersonalLanguageModelHelper.

Bug:20152986
Change-Id: I9d796fa57bf2073036bf11d86b143ff205a6199c