fix(cdk/overlay): hide native backdrop

Explicitly hides the native backdrop from the CDK popover so it doesn't get rendered out.

(cherry picked from commit bd52245)

Author: crisbeto

src/cdk/overlay/_index.scss

@@ -204,6 +204,11 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
     // with `align-self` can break the positioning (see #29809).
     inset: auto;
 
+    // For the time being we're using our `.cdk-overlay-backdrop` element instead of the native one.
+    &::backdrop {
+      display: none;
+    }
+
     .cdk-overlay-backdrop {
       position: fixed;
     }