refactor: move high-contrast style for button-toggle out of density

Moves the high-contrast style for standard button-toggles from the
density mixin to the base component styles. We previously moved the high
contrast style into the density mixin, but that is not stricly necessary
as the border width can be set to a value that is larger than common
button-toggles. The benefit is that this style can remain in the base
component styles and doesn't need to be generated for each density level.

This came up when we added backwards compatibility logic for the density
API. The high-contrast code uses `@at-root`, while the density API
intends to use that in some situations too. These nested `@at-root`
statements will not work as expected because the high contrast mixin
relies on the parent selector (ampersand) that is causing the density
`@at-root` to be a noop basically.

See: https://www.sassmeister.com/gist/b167ceb6b6d30c8c8c8db1580bce728d.

Author: devversion

src/material/button-toggle/_button-toggle-theme.scss

@@ -101,13 +101,6 @@
     .mat-button-toggle-appearance-standard .mat-button-toggle-label-content {
       line-height: $standard-height;
     }
-
-    @include cdk-high-contrast(active, off) {
-      .mat-button-toggle-checked.mat-button-toggle-appearance-standard
-      .mat-button-toggle-focus-overlay {
-        border-bottom: solid $standard-height;
-      }
-    }
   }
 }
 

src/material/button-toggle/button-toggle.scss

@@ -133,6 +133,17 @@ $mat-button-toggle-legacy-border-radius: 2px !default;
   }
 }
 
+@include cdk-high-contrast(active, off) {
+  .mat-button-toggle-checked.mat-button-toggle-appearance-standard
+  .mat-button-toggle-focus-overlay {
+    // In high contrast mode, we use a border for the checked state because backgrounds
+    // can either be opaque or transparent. We set the border height to a value that is larger
+    // than usual button toggles are. This allows us to keep this high contrast style in the
+    // base component style, instead of making it dependent on height determined through density.
+    border-bottom: solid 500px;
+  }
+}
+
 // Increase specificity because ripple styles are part of the `mat-core` mixin and can
 // potentially overwrite the absolute position of the container.
 .mat-button-toggle .mat-button-toggle-ripple {