Sync google3 changes.

 - fixes unicode feature computation
 - adds accepted configuration to the model to avoid running on unsupported
   text script
 - uses StringPiece for feature computation to improve effectivity

Test: Built and tested on device. Google3 unit and regression tests passing.

Bug: 36885478
Bug: 36886053
Change-Id: I3626831617b2480500e61c6951c2ee3e3c1b381d