Implement basic functionality for rust keyboard classifier

DD: go/project-imposter-android
This CL includes:
- Basic categorization into alphabetic and non-alphabetic
- Updating categorization based on key presses

Test: atest --host libinput_rust_test
Test: atest inputflinger_tests
Bug: 263559234
Flag: com.android.input.flags.enable_keyboard_classifier
Change-Id: I02bc2802233af69494753121cbeb091343b3f9cf