Fix missing @NonNull or @Nullable to TextClassifier APIs

FIX: 126703194
FIX: 126702907
FIX: 126701690
FIX: 126702936
FIX: 126702908
FIX: 126703116
FIX: 126700923
FIX: 126700363

Test: m update-api
Change-Id: I72add33844d4f7282c8bc178908ecc6cf1cb4a32