fix(material/core): theming validation for m2 themes firing incorrectly (#28707)

Fixes that theming validation for M2 themes was firing incorrectly, because the validation code was going through the `get-color-config`, `get-density-config` and `get-typography-config` functions which in turn were reading the internalized version of the theme where all the values were the same. Since Sass maps are immutable, they were being compared based on their values and always considered as the same theme.

(cherry picked from commit e524567)

Author: crisbeto

src/material/core/theming/_theming.scss

@@ -432,9 +432,10 @@ $_internals: _mat-theming-internals-do-not-access;
 // Checks for duplicate styles in a `theme-version: 0` style theme.
 @mixin _check-duplicate-theme-styles-v0($theme-or-color-config, $id) {
   $theme: private-legacy-get-theme($theme-or-color-config);
-  $color-config: get-color-config($theme);
-  $density-config: get-density-config($theme);
-  $typography-config: get-typography-config($theme);
+  $color-config: map.get($theme, $_internals, m2-config, color) or get-color-config($theme);
+  $density-config: map.get($theme, $_internals, m2-config, density) or get-density-config($theme);
+  $typography-config:
+    map.get($theme, $_internals, m2-config, typography) or get-typography-config($theme);
   // Lists of previous `color`, `density` and `typography` configurations.
   $previous-color: map.get($_emitted-color, $id) or ();
   $previous-typography: map.get($_emitted-typography, $id) or ();