Release the child layer on TaskSnapshot

Try to eliminante the offscreen layer from TaskSnapshot. When remove
the task snapshot window, although the SurfaceControl has destroyed in
WindowManager, but that layer seems still left as offscreen layer in
SurfaceFlinger, a possible reason is there didn't release the copied
layer so that reference keep it alive.

Test: keep hot launch apps and monitor no TaskSnapshot layer left in
offscreen layer.
Bug: 192298258

Change-Id: I6beff78112da2309d4825e185bd960974892f5e8