Update accessibility for popup

- Content description includes count of notifications.
- Notifications have a dismiss action.
- All icons that support shortcuts will have action to show
  shortcut menu, since there will be system shortcuts even if
  there are no deep shortcuts.

Bug: 36564782
Change-Id: I51b085fa26754f2dcd93c7db6548f2edf054f494

src/com/android/launcher3/notification/NotificationItemView.java

@@ -77,6 +77,10 @@ public class NotificationItemView extends PopupItemView implements LogContainerP
         mSwipeHelper.setDisableHardwareLayers(true);
     }
 
+    public NotificationMainView getMainView() {
+        return mMainView;
+    }
+
     public int getHeightMinusFooter() {
         int footerHeight = mFooter.getParent() == null ? 0 : mFooter.getHeight();
         return getHeight() - footerHeight;