3 state checkboxes should cycle thru all 3 states, not just enabled/disabled

[BretJohnson]

In VS, bring up the Android Designer.
Click, several times, on any of the checkboxes in the proppy UI for boolean values (not combinable/flags values).
In Android, those are all 3 state checkboxes as "unset" is a valid value, normally the default.

Result: Note as you click several times the checkbox flips between enabled & disabled, but never goes back to the unset set.

Expected: In current Mac android UI and the Android Studio UI, the checkboxes here cycle between all 3 states: unset, enabled, and disabled. We should do the same. That's arguably more user friendly, providing an easy and expected way for the user to undo the action of clicking on the checkbox.

[BretJohnson]

See https://github.com/xamarin/designer/pull/708 for another variant of this issue where it doesn't cycle at all, when a single checkbox in a combinable property (with no netural option) is checked and the user clicks on it.

Per @ermau

The underlying issue is the same, because we don't allow for an indeterminate value once the user has made a determinate input, we don't cycle to unset and we don't cycle to unset when the other third state is invalid.