Update MDC Select to use mdc-states mixin

[kfranqueiro]

Use the mdc-states mixin created in #1556

MDC Select will already use mdc-ripple-color once #1097 is merged, so wait for that first.

This should be replaced with the new mdc-states-color mixin, with any relevant opacity tweaks.

Note that it may not currently be feasible to support states on CSS-only selects due to browsers not giving native select elements ::before and ::after pseudo-elements. This might be possible if we add a later sibling element like what checkbox and radio use to support CSS-only.