Migrate DesktopTilingWindowDecoration to FocusTransitionObserver

This CL migrates DesktopTilingWindowDecoration from
ShellTaskOrganizer.FocusListener to FocusTransitionObserver.

Bug: b:381361555
Test: DesktopTilingWindowDecorationTest
Test: DesktopTilingWindowDecorViewModelTest
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions

Change-Id: Ib8564ef18d6f847ea1cdb17eeb8f38e457195226