bug(cdk-overlay-dark-backdrop): Is solid black in Windows high contrast dark mode

[haywoodsloan]

Reproduction

Steps to reproduce:

1. On a Windows machine enable high contrast mode with the black theme
2. Using Firefox navigate to: https://stackblitz.com/edit/angular-o7autm?file=src/app/./cdk-overlay-basic-example.html
3. Click the 'Open' button to show the overlay with backdrop enabled
4. See the overlay is solid black and obscures the rest of the page

Expected Behavior

The overlay does not obscure the rest of the page, such as the non-HC behavior:

Actual Behavior

The transparent overlay backdrop is converted to a solid black color and obscures everything else on the page:

Environment

• Angular: 80f4ff33381d37e72764d2ea3b7060e5952232e9
• CDK/Material: 70ff528
• Browser(s): Firefox
• Operating System (e.g. Windows, macOS, Ubuntu): Windows