Add dev option to force desktop mode

When turned on, this will:
- Enable system decorations on external screens
- Enable freeform windowing mode on external displays
- TODO: Show mouse pointer on external display

Bug: 112451761
Test: DisplaySettingsTests
Test: DesktopModePreferenceControllerTest
Test: FreeformWindowsPreferenceControllerTest
Change-Id: If46399b9d6b8faa2a11fbdf8a4bbfef1284147c8
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index d5dc903..9db7cf5 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -197,6 +197,7 @@
                     Settings.Global.DESK_DOCK_SOUND,
                     Settings.Global.DESK_UNDOCK_SOUND,
                     Settings.Global.DEVELOPMENT_ENABLE_FREEFORM_WINDOWS_SUPPORT,
+                    Settings.Global.DEVELOPMENT_FORCE_DESKTOP_MODE_ON_EXTERNAL_DISPLAYS,
                     Settings.Global.DEVELOPMENT_FORCE_RESIZABLE_ACTIVITIES,
                     Settings.Global.DEVELOPMENT_FORCE_RTL,
                     Settings.Global.DEVELOPMENT_SETTINGS_ENABLED,