Do not update task layer during collecting.

Which can prevents task layer be updated by pending transaction, no
matter that task has participant the transition. So the layer change
can always align with shell transition.
Then when transition ready, update the layers to start and finish
transaction, and correct the layer of transition roots after all layers
are assigned, so the last layer would be correct even if the reference
root window wasn't allow to assign layer by any reason.

Bug: 291510956
Test: tried on sample app, verify no flickering when close the task
with finishIfPossible or startActivityFromResult.

Change-Id: I4ccaca494780a636bd65f695bc9c9d9af9c7e5a4