DisplayTopology: user cannot drag only display

When dragging a single display, we don't constrain the position of the
display, and the user can drag it anywhere on the fragment. As such we
need to either prevent the drag or refresh the pane always if there is
only one display. This CL chooses the former.

Flag: com.android.settings.flags.display_topology_pane_in_display_list
Test: atest DisplayTopologyPreferenceTest.kt
Bug: b/352650922
Change-Id: Icb101b734ce9b88435f64a71bf77f878f9b230e0