Topology pane in extended displays list

Show an empty topology pane in the extended displays list. Even show
when no external display is connected.

This will be tweaked in a follow-up CL to include the built-in display
in the pane and the displays list.

This fixes an issue where the displays list is not shown when *no*
external displays are connected, which was not the intent of the flag.
The previous flag CL ag/30358161 only respected the flag when 1 external
display was connected.

Based on mocks at go/al-mm-figma

Bug: b/352650922
Test: reboot with zero displays attached and verify display list and pane are shown
Test: on al-13 device, make the activity fullscreen and verify the margins on left and right of pane are larger
Flag: com.android.settings.flags.display_topology_pane_in_display_list
Change-Id: If39fefe943a26c817fa6f636f21eb8aaa080adde