Ensure external display to be freeform display

This CL ensures that an external display is always a freeform display
when the desktop mode is supported.
As enableDisplayContentModeManagement flag deprecated
DEVELOPMENT_FORCE_DESKTOP_MODE_ON_EXTERNAL_DISPLAYS, which previously
affected the windowing mode of external displays.
DesktopDisplayModeController should be responsible for it instead now.

Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Bug: 404724976
Test: DesktopDisplayModeControllerTest
Test: DesktopDisplayEventHandlerTest
Change-Id: I9284a20f7de49a6cfc0a61f96b0fd793984dab38