Disallow reparenting into a leaf task.

Allowing task to task reparenting ends up reparenting under a leaf task
for certain cts tests.

Bug: 434499068
Test: atest CtsWindowManagerDeviceWindow:MultiWindowTests & observe that
there is no exception thrown
Flag: com.android.window.flags.enable_desktop_windowing_enterprise_bugfix

Change-Id: I0727dacf950ea113480e456c9fb014f77e53e3c3