text input: Reset hints when text input gone

This ensures that when the keyboard is force brought up next time, the user is not stuck with an overlay they can't switch.

As a downside, there's a blink of default hint before the keyboard hides.

This fixes https://source.puri.sm/Librem5/squeekboard/-/issues/256

Not sure what a "proper" solution for this should be, but trading a functional bug for a cosmetic one that people may rarely notice counts as a win for me.

I submitted another approach here https://source.puri.sm/Librem5/squeekboard/-/merge_requests/421