fix(cdk/overlay): temporarily hide getAttachedOverlays

We're still trying to figure out the best way to expose `getAttachedOverlays`. Hide them from the API for now to avoid the breaking change later.

Author: crisbeto

goldens/cdk/overlay/index.api.md

@@ -332,9 +332,6 @@ export class FullscreenOverlayContainer extends OverlayContainer implements OnDe
     static ɵprov: i0.ɵɵInjectableDeclaration<any>;
 }
 
-// @public
-export function getAttachedOverlays(): OverlayRef[];
-
 // @public
 export class GlobalPositionStrategy implements PositionStrategy {
     apply(): void;

src/cdk/overlay/public-api.ts

@@ -20,7 +20,7 @@ export {
   CDK_CONNECTED_OVERLAY_DEFAULT_CONFIG,
 } from './overlay-directives';
 export {FullscreenOverlayContainer} from './fullscreen-overlay-container';
-export {OverlayRef, OverlaySizeConfig, getAttachedOverlays} from './overlay-ref';
+export {OverlayRef, OverlaySizeConfig} from './overlay-ref';
 export {ViewportRuler} from '../scrolling';
 export {ComponentType} from '../portal';
 export {OverlayPositionBuilder} from './position/overlay-position-builder';