chore(slider): Refactor thumb interaction handlers (#1804)
chore(slider): Refactor thumb interaction handlers (#1804)

Expose thumb interaction functionality as methods on the foundation. By exposing the methods, porting the library to other frameworks will be easier. Yay! Rename methods from `onDown`, `onMove`, and `onUp` to `handleDown_`, `handleMove_`, and `handleUp_`. Remove unnecessary `createDownHandler_` in favor of a simple arrow function.
material-components/material-components-web,material-components/material-components-web,material-components/material-components-web