Fix enter desktop transition flicker in BD2A build

There's an extra home change in enter desktop transition info in BD2A
builds (b/368334295).
- It helped to uncover that the dragged layer z-order is being reset for the 2nd (merged) transition and puts it behind wallpaper task, effectively hiding it. It's fixed by restoring the dragged layer z-order to the one assigned in the first transition.
- It also produced an unwanted consequence. Currently we expect only 1 home change in transition info, but we receive more. This CL filters only for the visible home task and uses it for the animation.

Bug: 361641893
Test: manual, atest WMShellUnitTests:DragToDesktopTransitionHandlerTest
Flag: EXEMPT bugfix
Change-Id: Ic08d55e123ad89ff3f5478b73560f56c8f46339b