canEnterDesktopMode is only true if desktop mode can be started.

ag/33290267 corrected isDeviceEligibleForDesktopMode to allow the
developer option to be enabled. But doing so, it enabled desktop mode
on all devices that support projected mode, while it's not launched yet.

Fix: 415214580
Bug: 389092752
Flag: com.android.window.flags.enable_projected_display_desktop_mode
Test: Manual
Test: atest DesktopModeStatusTest DesktopStateImplTest
Change-Id: Ib799ee486b8f52108cd753cc3c6ca0eb354383d5