System decoration-based extended mode check

This CL changes the extended mode check to use the availability of
system decorations.
With ENABLE_DISPLAY_CONTENT_MODE_MANAGEMENT, it is the source of truth
of the display “content mode” (i.e., extended or mirror).

Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Flag: com.android.window.flags.enable_display_windowing_mode_switching
Bug: 375319538
Test: DesktopDisplayModeControllerTest
Change-Id: I29885eff6252dd8ffdcd69511ad8017ae2388c03