Copying across latest version of libtextclassifier

Changes include size reductions by removing protos and
proto fields. Also a bug fix in the feature-processor
which didn't affect the inference code path.

Test: libtestclassifier_tests pass in google3 repo.
Bug: 34865247
Change-Id: I010ca9fc5b6ccf68aa8701bbd5bee6bea259d06b