Add basic integration tests for shelf

This adds tests for the behaviour of the notification shelf when there is an overflow of notifications.

Test: atest ShelfInvisibleWithLowNotifications && atest ShelfVisibleAtTheBottom
Bug: 227733494
Change-Id: I3041d58fd70155069265a589eefd4a051fe6ac11
diff --git a/libraries/app-helpers/interfaces/handheld/src/android/platform/helpers/INotificationHelper.java b/libraries/app-helpers/interfaces/handheld/src/android/platform/helpers/INotificationHelper.java
index 294de7b..3551678 100644
--- a/libraries/app-helpers/interfaces/handheld/src/android/platform/helpers/INotificationHelper.java
+++ b/libraries/app-helpers/interfaces/handheld/src/android/platform/helpers/INotificationHelper.java
@@ -136,6 +136,15 @@
     }
 
     /**
+     * @return the notification shelf, containing overflow notifications that don't fit in the
+     *     screen. If unavailable, returns null.
+     */
+    @Nullable
+    default UiObject2 getNotificationShelf() {
+        throw new UnsupportedOperationException("Not yet implemented.");
+    }
+
+    /**
      * Setup Expectations: Shade is open
      *
      * <p>Posts multiple bubble notification. These notifications are associated with a conversation