fix(material/expansion): move unthemable tokens to theme mixin (#27885)

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)

(cherry picked from commit 52c85e5)

Author: mmalerba

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;