Do not update root task layer without transition

All root tasks are organized, so the layer should only be set
with a transition.

Bug: 385142011
Flag: com.android.window.flags.update_root_task_layer_only_with_transition
Test: atest ZOrderingTests
Change-Id: I7e4c3bdae8fbff96e21c43f949f0acdecadc8919