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