fix(material/snack-bar): 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)

BREAKING CHANGE:
There are new styles emitted by `mat.snack-bar-theme` that are not
emitted by any of: `mat.snack-bar-color`, `mat.snack-bar-typography`,
`mat.snack-bar-density`. If you rely on the partial mixins only and don't
call `mat.snack-bar-theme`, you can add `mat.snack-bar-base` to get the
missing styles.

Author: amysorto

src/material/_index.scss

@@ -167,7 +167,7 @@
 @forward './slider/slider-theme' as slider-* show slider-theme, slider-color, slider-typography,
   slider-density;
 @forward './snack-bar/snack-bar-theme' as snack-bar-* show snack-bar-theme, snack-bar-color,
-  snack-bar-typography, snack-bar-density;
+  snack-bar-typography, snack-bar-density, snack-bar-base;
 @forward './legacy-snack-bar/snack-bar-theme' as legacy-snack-bar-* show legacy-snack-bar-theme,
   legacy-snack-bar-color, legacy-snack-bar-typography;
 @forward './sort/sort-theme' as sort-* show sort-theme, sort-color, sort-typography, sort-density;

src/material/snack-bar/_snack-bar-theme.scss

@@ -1,10 +1,17 @@
 @use '@material/snackbar/snackbar-theme' as mdc-snackbar-theme;
 @use '../core/theming/theming';
+@use '../core/style/sass-utils';
 @use '../core/typography/typography';
 @use '../core/tokens/token-utils';
 @use '../core/tokens/m2/mdc/snack-bar' as tokens-mdc-snack-bar;
 @use '../core/tokens/m2/mat/snack-bar' as tokens-mat-snack-bar;
 
+@mixin base($config-or-theme) {
+    // Add default values for tokens not related to color, typography, or density.
+  @include sass-utils.current-selector-or-root() {
+    @include mdc-snackbar-theme.theme(tokens-mdc-snack-bar.get-unthemable-tokens());
+  }
+}
 
 @mixin color($config-or-theme) {
   $config: theming.get-color-config($config-or-theme);
@@ -36,6 +43,7 @@
     $density: theming.get-density-config($theme);
     $typography: theming.get-typography-config($theme);
 
+    @include base($theme);
     @if $color != null {
       @include color($color);
     }

src/material/snack-bar/snack-bar-container.scss

@@ -47,7 +47,6 @@
     // surface element.
     @include mdc-snackbar-theme.elevation(mdc-snackbar-theme.$elevation);
     @include mdc-snackbar-theme.theme-styles(tokens-mdc-snack-bar.get-token-slots());
-    @include mdc-snackbar-theme.theme(tokens-mdc-snack-bar.get-unthemable-tokens());
 
     // MDC sets the position as fixed and sets the container on the bottom center of the page (or
     // otherwise can be set to be "leading"). Our overlay handles a more advanced configuration