InputVerifier: make action_button a field of the action enum

Only MotionEvents with a button press or release action should have an
action button. Let's express this in the data structure by making
action_button a field of the relevant MotionActions, instead of a
separate field of MotionEvent that could potentially be set on
non-button actions.

This involves moving the translation from input_bindgen constants to
MotionAction into lib.rs, which I think makes more sense (as it keeps
the language interfacing code contained there), but also means we have
to move one piece of validation there too.

Bug: 245989146
Test: $ atest --host libinput_rust_test
Test: enable the verifier, check everything works as usual
Flag: EXEMPT refactor
Change-Id: Ida17429b0e12247b63a3ae44bab63e421d9fff0f