Small native refactoring.

Move a purely dictionary-format-related function that is needed
both by unigrams and bigrams to the binary format handling
file.
Also remove the empty UnigramDictionary::getBigrams placeholder
function, on grounds that it should be in the BigramDictionary
class.

Bug: 5046459
Change-Id: I8a67a25f72122e2fa0b19ae1d936db25eb0b20ba