Introduce scrollbar in widget tray

b/21270754

Change-Id: I82dd87193f0ed5822f8cd75ef0c785057f949015
diff --git a/res/layout/widgets_view.xml b/res/layout/widgets_view.xml
index 9d9fa10..196dc45 100644
--- a/res/layout/widgets_view.xml
+++ b/res/layout/widgets_view.xml
@@ -39,6 +39,7 @@
             android:layout_width="match_parent"
             android:layout_height="match_parent"
             android:background="@drawable/quantum_panel_dark"
+            android:scrollbars="vertical"
             android:elevation="15dp"
             android:visibility="gone" />