fix(datepicker): screenreaders report editable grid cells (#12275)

* Since the `aria-readonly` attribute is not set properly, in Firefox with NVDA, the screenreader announces that every date (grid cell) is editable. This is not true, because those cells are just selectable.

src/lib/datepicker/calendar-body.ts

@@ -42,7 +42,7 @@ export class MatCalendarCell {
   host: {
     'class': 'mat-calendar-body',
     'role': 'grid',
-    'attr.aria-readonly': 'true'
+    'aria-readonly': 'true'
   },
   exportAs: 'matCalendarBody',
   encapsulation: ViewEncapsulation.None,