Mark Layer as modified when changing zOrderRelativeOf.

If the layer is not marked as modified, the changes will not get copied
from current to drawing. This will break drawing state since it will be
missing info about what a layer is currently relativeOf.

Fixes: 125468401
Test: Can no longer repro issue in bug.
Change-Id: Id75a2bc87c179c4bd11141032acc612e08f0db7b
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 046482c..3763c58 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -899,6 +899,8 @@
 
     // Layer bounds in screen space.
     FloatRect mScreenBounds;
+
+    void setZOrderRelativeOf(const wp<Layer>& relativeOf);
 };
 
 } // namespace android