Improve backward compatibility of plurals formatting.

icu4c's behavior changed such that it now looks at the absolute value of the
input. For now, preserve compatibility by treating all negative numbers as
being case OTHER.

Bug: 18429565
Bug: https://code.google.com/p/android/issues/detail?id=78962
Change-Id: I1e98af973f408c8809071c15d07d4fd585d77e4c