fix(expansion): incorrect margin for "before" indicator when expanded (#16420)

Fixes the panel header indicator not being positioned correctly when it's set to the `before` position and it's expanded. The issue comes from the fact that we were using `padding` instead of `margin` which changes the center of the rotation.

Also fixes that the `before` position of the indicator doesn't account for RTL.

Author: crisbeto

src/material/expansion/expansion-panel-header.scss

@@ -23,7 +23,11 @@
     flex-direction: row-reverse;
 
     .mat-expansion-indicator {
-      padding: 0 16px 0 0;
+      margin: 0 16px 0 0;
+
+      [dir='rtl'] & {
+        margin: 0 0 0 16px;
+      }
     }
   }
 }