Improve post-assignment dialog for Answer actions in Controls

_Originally posted by @ZornHadNoChoice in [#20504](https://github.com/ankidroid/Anki-Android/issues/20504#issue-4086046411)_:

> Not sure if this counts as a bug, but I noticed in https://github.com/ankidroid/Anki-Android/pull/20701 that the dialog which asks the user if they want the gesture to be "Only answer" or "Flip and answer" does nothing when the gesture is already assigned to "Show answer". Possible solutions:
>
> 1. The gesture is removed from "Show answer" if the user selects "Only answer".
> 2. The dialog isn't shown at all in this case.
> 3. Change the wording of the dialog to something like "Also assign to Show answer?" and have "Yes"/"No" buttons.

---

Current dialog, for reference: