fix(material-experimental/theming): cap icon button size

Fixes that the icon button was becoming smaller than its icon at the lowest density.

Author: crisbeto

src/material-experimental/theming/_m3-density.scss

@@ -41,7 +41,8 @@ $_density-tokens: (
     container-height: (40px, 36px, 32px, 28px),
   ),
   (mdc, icon-button): (
-    state-layer-size: (40px, 36px, 32px, 28px, 24px, 20px),
+    // The size caps out at 24px, because anything lower will be smaller than the icon.
+    state-layer-size: (40px, 36px, 32px, 28px, 24px, 24px),
   ),
   (mdc, linear-progress): (),
   (mdc, list): (