fix(checkbox): Fix background fading too fast (#2122)

Fix the checked to unchecked background fade animation which was causing the background to abruptly disappear instead of fade nicely.

packages/mdc-checkbox/constants.js

@@ -43,7 +43,7 @@ const strings = {
 
 /** @enum {number} */
 const numbers = {
-  ANIM_END_LATCH_MS: 100,
+  ANIM_END_LATCH_MS: 250,
 };
 
 export {cssClasses, strings, numbers};