Use new enableDesktopWindowingEnterTransitions flag

Enable enter-transitions either when the flag
- enableDesktopWindowingTransitions, or the flag
- enableDesktopWindowingEnterTransitions
is enabled.

Use the DesktopModeFlags field for enableDesktopWindowingEnterTransitions.

Test: n/a
Bug: 369763947
Flag: com.android.window.flags.enable_desktop_windowing_enter_transitions
Change-Id: I7a12c6e6d5c125c4d9739c7cddc2742e11da8b9d