[Checkbox] Checkbox fills different color when animating

[abhiomkar]

Bug report

Checkbox fills container color with previous component override color.

Steps to reproduce

1. Go to https://glitch.com/~lovely-beret demo
2. Check & uncheck the first checkbox

Actual behavior

Observe that the first checkbox fills color of second checkbox

Expected behavior

The checkbox container color should be scoped to component instance when mdc-checkbox-container-colors is used.

Your Environment

Software	Version(s)
MDC Web	v2.1.1
Browser	any
Operating System	any

Possible solution

The keyframes in mdc-checkbox-container-colors mixin implementation are being generated outside the scope of custom checkbox class, it should be scoped.