refactor(datepicker): export internal multi-year view symbols

We need to export the `MatMultiYearView` symbol in the
`@angular/material/datepicker` entry-point in order to be able
to build a flat-module entry with Ivy ngtsc. Ngtsc enforces that
this class needs to be explicitly exported as it is part of the
public exported module `MatDatepickerModule`.

In order to fix this failure we just export the class similar to
other internal components such as `MatYearView`.

src/material/datepicker/public-api.ts

@@ -16,3 +16,4 @@ export * from './datepicker-intl';
 export * from './datepicker-toggle';
 export * from './month-view';
 export * from './year-view';
+export * from './multi-year-view';

tools/public_api_guard/material/datepicker.d.ts

@@ -251,6 +251,29 @@ export declare class MatMonthView<D> implements AfterContentInit {
     ngAfterContentInit(): void;
 }
 
+export declare class MatMultiYearView<D> implements AfterContentInit {
+    _dateAdapter: DateAdapter<D>;
+    _matCalendarBody: MatCalendarBody;
+    _selectedYear: number | null;
+    _todayYear: number;
+    _years: MatCalendarCell[][];
+    activeDate: D;
+    readonly activeDateChange: EventEmitter<D>;
+    dateFilter: (date: D) => boolean;
+    maxDate: D | null;
+    minDate: D | null;
+    selected: D | null;
+    readonly selectedChange: EventEmitter<D>;
+    readonly yearSelected: EventEmitter<D>;
+    constructor(_changeDetectorRef: ChangeDetectorRef, _dateAdapter: DateAdapter<D>, _dir?: Directionality | undefined);
+    _focusActiveCell(): void;
+    _getActiveCell(): number;
+    _handleCalendarBodyKeydown(event: KeyboardEvent): void;
+    _init(): void;
+    _yearSelected(year: number): void;
+    ngAfterContentInit(): void;
+}
+
 export declare class MatYearView<D> implements AfterContentInit {
     _dateAdapter: DateAdapter<D>;
     _matCalendarBody: MatCalendarBody;
@@ -273,3 +296,7 @@ export declare class MatYearView<D> implements AfterContentInit {
     _monthSelected(month: number): void;
     ngAfterContentInit(): void;
 }
+
+export declare const yearsPerPage = 24;
+
+export declare const yearsPerRow = 4;