external displays: mirror/extend switch

Add a mirror/extend built-in display switch. Make minor changes to
DisplayTopology.kt for consistency and correctness.

Kotlin requires the two preference key names are different since they
are in the same namespace, so fix the name in the existing
DisplayTopology.kt module.

Make DisplayTopologyPreference responsible, rather than the caller, for
setting its persistence property, since a wrong value may cause unusual
behavior.

The setOrder calls are necessary to prevent the new switch from
appearing below the Built-in display category when a display is
hot-plugged in after showing the UI. We set them on all top-level
preferences (not just the two we are fixing) for consistency.

Flag: com.android.settings.flags.display_topology_pane_in_display_list
Test: atest ExternalDisplayPreferenceFragmentTest.java
Bug: b/352648432
Bug: b/366056921
Change-Id: Ib0072dd75066758903cc48c2d1e7142e1d921f67

src/com/android/settings/connecteddevice/display/DisplayTopology.kt

@@ -157,7 +157,7 @@ class TopologyScale(
     }
 }
 
-const val PREFERENCE_KEY = "display_topology_preference"
+const val TOPOLOGY_PREFERENCE_KEY = "display_topology_preference"
 
 /** Padding in pane coordinate pixels on each side of a display block. */
 const val BLOCK_PADDING = 2f
@@ -220,7 +220,8 @@ class DisplayTopologyPreference(context : Context)
         // Prevent highlight when hovering with mouse.
         isSelectable = false
 
-        key = PREFERENCE_KEY
+        key = TOPOLOGY_PREFERENCE_KEY
+        isPersistent = false
 
         injector = Injector(context)
     }