$cdk-z-index-overlay-* scss variables should have !default

[Totati]

What is the expected behavior?

Being able to configure the $cdk-z-index-overlay, $cdk-z-index-overlay-container, and $cdk-z-index-overlay-backdrop values.

What is the current behavior?

I have to override the values with css selectors.

What are the steps to reproduce?

$cdk-z-index-overlay-container: 1080;
$cdk-z-index-overlay: 1080;
$cdk-z-index-overlay-backdrop: 1080;
@import '~node_modules/@angular/cdk/overlay';
@include cdk-overlay();

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

"@angular/animations": "~7.2.0",
"@angular/cdk": "^7.3.3",
"@angular/common": "~7.2.0",
"@angular/compiler": "~7.2.0",
"@angular/core": "~7.2.0",
"@angular/forms": "~7.2.0",
"@angular/platform-browser": "~7.2.0",
"@angular/platform-browser-dynamic": "~7.2.0",
"@angular/router": "~7.2.0",
"@angular/material": "^7.3.3",
"@ngx-translate/core": "~11.0.1",
"bootstrap": "4.2.1",
"core-js": "^2.5.4",
"ngx-bootstrap": "^3.1.4",
"rxjs": "~6.3.3",
"tslib": "^1.9.0",
"zone.js": "~0.8.26"

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