fix(material/expansion): move unthemable tokens to theme mixin

Though these tokens are not currently affected by the theme, in the
future they will be affected by the design system used for theming (M2
or M3)

src/material/expansion/_expansion-theme.scss

@@ -6,7 +6,10 @@
 @use '../core/tokens/m2/mat/expansion' as tokens-mat-expansion;
 
 @mixin base($theme) {
-  // TODO(mmalerba): Move expansion panel base tokens here
+  @include sass-utils.current-selector-or-root() {
+    @include token-utils.create-token-values(
+        tokens-mat-expansion.$prefix, tokens-mat-expansion.get-unthemable-tokens());
+  }
 }
 
 @mixin color($theme) {

src/material/expansion/expansion-panel.scss

@@ -6,8 +6,6 @@
 @use '../core/style/elevation';
 
 .mat-expansion-panel {
-  @include token-utils.create-token-values(
-    tokens-mat-expansion.$prefix, tokens-mat-expansion.get-unthemable-tokens());
   @include elevation.overridable-elevation(2);
   box-sizing: content-box;
   display: block;