Checkbox: Add on-background property and deprecate dark property

[daneah]

The problem
Many components can be asked to display inverse-style colors using an on-background property. The odd one out is Checkbox, which uses a dark property. The on-background property should be added and preferred instead, for both consistency and decoupling from equating the concept of "dark" to the concept of "on-background."

The solution

• Add an on-background property that mirrors the current behavior of dark
• Mark dark as deprecated