Do not play enter dim animation when starting window is on top of task.

There does not need to play the entering dim animation when starting
window is exist in the task, since the starting window should always
above the dim layer.
And allows to apply dim if the host is the task itself, because it
won't affect display anyway.

Bug: 295280282
Test: verify less flickering when opening a dimming task from recents.
Change-Id: I92ae7e15d483bbd29d17c9c1e14d27a2cc735c91