Fix hotseat visibility after dragging task to bubble

Desktop drag handler sends a transition at the beginning to transiently
launch home task. This transition is ignored by the home transition
observer and we do not update home visibility in this case. As this
would otherwise cause flickers with launcher UI elements during drag.

When the desktop drag ends in a bubble drag zone, we send a new
transition to convert the task to a bubble. If the task was the only
task running on home, we now will have home showing and the bubbled task
showing. But since home was made visible by the drag start transition,
home task is not part of the second transition changes. It is already
considered visible.
This broke the existing logic with handling home visibility as we were
expecting the second transition to include information about the home
task.
As a fix, keep track of the desired home visibility state when desktop
drag starts. And if the subsequent transition to convert the task to
bubble does not contain information about home, apply the home
visibility that was calculated at the start of drag.
If the convert to bubble transition does include home task and it
triggers a change to home visibility, apply this instead and discard the
home visibility pending update from the start of the drag.
This second case is required in case there are two tasks running on top
of home. When desktop drag starts, home will be brought to front
transiently. And when drag ends with a transition to convert the dragged
task to bubble, the transition changes will include a change to move the
home task behind the second fullscreen task. We need to honor this case
and send an update that home is not visible.

Bug: 396475362
Flag: com.android.wm.shell.enable_bubble_to_fullscreen
Test: atest HomeTransitionObserver
Test: manual, open an app, drag it to a bubble, check that hotseat is
  shown in launcher
Change-Id: I197ac9b96c4312f3da12bdad21b5844b414847c2