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
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 44e60ed..699c85c 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -1594,7 +1594,7 @@
     return true;
 }
 
-void Layer::reparentChildrenForDrawing(const sp<Layer>& newParent) {
+void Layer::setChildrenDrawingParent(const sp<Layer>& newParent) {
     for (const sp<Layer>& child : mDrawingChildren) {
         child->mDrawingParent = newParent;
     }