Move flags belonging to BinaryFormat to the right place.

These masks and flags are constants that are an integral part
of the format. They belong in BinaryFormat and have nothing to
do in UnigramDictionary.
This needs I6751dda4 to not break the build

Bug: 6429243
Change-Id: Ic1c842b3245f7fdc25aa8d1459c5bb07b262e265