Add flag for surfacing display topology prototype

This will force the topology pane to show in the displays list UI, and
make the displays list UI accessible from the connected devices
fragment, regardless of the number of actual devices connected.

Once the display topology pane is using real data, this will still be
useful to not skip the display list UI, which currently can only be
shown when there is more than one external display connected.

To turn off, use:
$ adb shell device_config put display_manager com.android.settings.flags.display_topology_pane_in_display_list false

Test: verify "External Display" is always shown in connected devices list
Test: verify displays list is shown when 0 or 1 external display is connected
Bug: b/352648432
Flag: com.android.settings.flags.display_topology_pane_in_display_list
Change-Id: Ic9205c69d9de09e6fb8ae33e17fbf00fe5574447