Small fixes for TextClassifierImpl

- Incorrect nullable annotation for Options
- Bad model output validation (start==end would pass, causing crashes in
    TextSelection constructor.
- Fix for potential null pointer exceptions in intent label generation.
- Fixed missing label for adding to contacts

Test: Manually verified add contact intent
Change-Id: I4ce0fdd1b1826caa11050a0dc808c9f8b28f90c2