Add a separate line at the bottom of list area in output switcher

-Bottom divider is not showed when there are more than 7 devices exist in output switcher
-Check panel type in isDividerAllowedBelow()
-Hide the Slice bottom divider of Output switcher panel
-Show divider in panel layout only for output switcher panel

Bug: 159177275
Test: make -j42 RunSettingsRoboTests

Change-Id: I49396a0ee0543b494a2b3d714ec248ebab08e7ba

res/layout/panel_layout.xml

@@ -86,6 +86,11 @@
         <!-- Note: There is a landscape version of panel_slice_list which supports scrolling. -->
         <include layout="@layout/panel_slice_list"/>
 
+        <include
+            android:id="@+id/footer_divider"
+            layout="@layout/horizontal_divider"
+            android:visibility="gone"/>
+
         <LinearLayout
             android:layout_width="match_parent"
             android:layout_height="wrap_content"