Put window decors at one place

There are some juggling between task listeners and the transition
observer of window decors because they are hosts together with its
corresponding task states. This change can remove those juggling by
putting all of them at one place.

Bug: 250622579
Test: atest WMShellUnitTests
Change-Id: Ic1db1633a5e79e2bcea5885b8fc2f345fa05fb1e