chore(overlay): export OverlayKeyboardDispatcher to aide in extending the Overlay class. (#8311)

Author: josephperrott

src/cdk/overlay/public-api.ts

@@ -17,6 +17,7 @@ export {FullscreenOverlayContainer} from './fullscreen-overlay-container';
 export {OverlayRef} from './overlay-ref';
 export {ViewportRuler} from '@angular/cdk/scrolling';
 export {ComponentType} from '@angular/cdk/portal';
+export {OverlayKeyboardDispatcher} from './keyboard/overlay-keyboard-dispatcher';
 
 // Export pre-defined position strategies and interface to build custom ones.
 export {PositionStrategy} from './position/position-strategy';