WindowManager: Avoid reparenting BLAST Surface in reparentChildren

In cases where we are replacing the client surface transparently (e.g.
preserved surfaces), we call reparentChildren to move client added
Surfaces of the preserved Surface to the new Layer. However we are calling
this on the WSA layer, and so we end up reparenting the BLAST Surface
to the new layer. But since we always construct a new BLAST surface on the
WM side, we end up with 2! Rather than try and juggle about when we need
to or need not to construct a BLAST surface and reparenting them across
preservedSurfaces, it seemed either to just use the BLAST surface as the
root of the reparentChildren operation.

Bug: 150013915
Test: Enable BLAST, use split-screen
Change-Id: I1357200b41d183c2331b684ff636dd40a3b98168