SurfaceFlinger: Fix removal check when adding child layers.

Traversing the current state has the same problem we had in other
CL's with relative Z. In this case if there was a parent that
was relative-Zed then it's Z relative is removed, we will be unable
to add children to it until we assign it a new layer. However there's
no reason to traverse, we can just use the new "isPendingRemoval"
flag to detect this state.

Bug: 69633137
Test: Existing tests pass.
Change-Id: I4584f051346fe77c3e6158a04dd1279a86164476