omit helper func from public api

Author: jysoo

src/material/datepicker/public-api.ts

@@ -16,4 +16,4 @@ export * from './datepicker-intl';
 export * from './datepicker-toggle';
 export * from './month-view';
 export * from './year-view';
-export * from './multi-year-view';
+export {MatMultiYearView, yearsPerPage, yearsPerRow} from './multi-year-view';