fix(datepicker): backdrop class should be mat- (#7056)

Author: andrewseguin

src/lib/datepicker/datepicker.ts

@@ -335,7 +335,7 @@ export class MdDatepicker<D> implements OnDestroy {
     const overlayState = new OverlayConfig({
       positionStrategy: this._createPopupPositionStrategy(),
       hasBackdrop: true,
-      backdropClass: 'md-overlay-transparent-backdrop',
+      backdropClass: 'mat-overlay-transparent-backdrop',
       direction: this._dir ? this._dir.value : 'ltr',
       scrollStrategy: this._scrollStrategy()
     });