fix(material/radio): Apply tokens at mixin root

Applies radio tokens at the theme mixin's root selector (or html if the mixin
is called with no selector). This makes it easier for users to override tokens
without worrying about specificity.

Author: mmalerba

src/material/radio/_radio-theme.scss

@@ -19,7 +19,7 @@
 
 @mixin color($theme) {
   @include mdc-helpers.using-mdc-theme($theme) {
-    .mat-mdc-radio-button {
+    @include sass-utils.current-selector-or-root() {
       @include mdc-form-field.core-styles($query: mdc-helpers.$mdc-theme-styles-query);
     }
   }