Add DesktkopState#isProjectedMode.

Helper method for other parts of Shell, Launcher to quickly decide
whether the device is currently in projected mode or not.

Bug: 410592525
Test: atest
Flag: com.android.window.flags.enable_projected_display_desktop_mode
Change-Id: Ifb621b572e348e729231b4e27e32fc0ba41e323b