fix(cdk/a11y): make cdk-high-contrast work w/ emulated view encapsulation

Say you have this in your component:
```scss
.some-element {
  @include cdk-high-contrast() {
    border: 1px solid white;
  }
}
```

With this change, this will output:
```scss
.cdk-high-contrast-active .some-element,
.cdk-high-contrast-active :host .some-element {
  border: 1px solid white;
}
```

Here, `.cdk-high-contrast-active .some-element` will apply in places
where encapsulation is turned off, and `.cdk-high-contrast-active :host
.some-element` will apply in cases where encapsulation is emulated.
Neither will work in Shadow DOM (which we don't officially support).
AFAIK, Shadow DOM would need to use `:host-content()`, which we could
consider adding if we get an additional request.

This adds a few more bytes, but high-contrast styles tend to be pretty
limited.

Author: jelbourn

src/cdk/a11y/_a11y.scss

@@ -35,7 +35,8 @@
   // context. We address this by nesting the selector context under .cdk-high-contrast.
   @at-root {
     $selector-context: #{&};
-    .cdk-high-contrast-#{$target} {
+    .cdk-high-contrast-#{$target},
+     .cdk-high-contrast-#{$target} :host {
       @if ($selector-context == '') {
         @content;
       }