fix(list): properly align contents in subheader

* The line-height of the subheaders is currently based on the typography level. This is problematic because the subheader is set to a specific height and can't grow/shrink accordingly.

Fixes angular#6214

src/lib/list/_list-theme.scss

@@ -46,7 +46,8 @@
     }
 
     .mat-subheader {
-      @include mat-typography-level-to-styles($config, body-2);
+      font: mat-font-weight($config, body-2) mat-font-size($config, body-2)
+            mat-font-family($config, body-2);
     }
   }
 

src/lib/list/list.scss

@@ -121,6 +121,7 @@ $mat-dense-list-icon-size: 20px;
 // This mixin adjusts the heights and padding based on whether the list is in dense mode.
 @mixin mat-subheader-spacing($top-padding, $base-height) {
   height: $base-height;
+  line-height: $base-height - $mat-list-side-padding * 2;
 
   &:first-child {
     margin-top: -$top-padding;