fix(material/menu): inconsistent layout of submenu icon

* Fixes that the offset of the submenu icon wasn't identical between LTR and RTL.
* Fixes that the layout of a menu with sub-menu was different if the app uses `box-sizing: border-box`.

Fixes #29599.

Author: crisbeto

src/material/core/style/_menu-common.scss

@@ -63,6 +63,7 @@ $icon-margin: 16px !default;
   width: $icon-size;
   height: 10px;
   fill: currentColor;
+  box-sizing: content-box;
 
   // We use `padding` here, because the margin can collapse depending on the other content.
   padding-left: $item-spacing;
@@ -74,6 +75,7 @@ $icon-margin: 16px !default;
     // Invert the arrow direction.
     polygon {
       transform: scaleX(-1);
+      transform-origin: center;
     }
   }