SurfaceFlinger: Fix reparenting following ReparentForDrawing

Counterintuitively, the new-parent doesn't have the children in
it's child list so even when cleaning up we need to call
reparentChildrenForDrawing on the old parent instead of the new parent.
Also clarify name to make this more obvious and add test.

Test: Transaction_test.cpp
Bug: 76099859
Change-Id: I935eec303dba320b5a2731606ec862311c5ca43e