Reduce unnecessary collecting when starting on existing task

It was added for moving a top activity to a different display:
Ib4949721eb77fb96eaa19e210f97b44163d4e4a5
It can be removed since the general display change has covered this:
Iba774557bb5ed8774d8e6b5c0bc72e8761628da6
  recycleTask > setTargetRootTaskIfNeeded
     > getOrCreateRootTask > reparent > onDisplayChanged

That avoids the additional collecting if there is no change to
the activity, e.g. launch a dialog activity on the same task.

Bug: 373410849
Flag: EXEMPT bugfix
Test: atest TransitionTests#testCollectReparentChange
Change-Id: I4d6e6d069c9e5fca3730c83bc75f312c7395052c