SurfaceFlinger and libgui: Support for child layers.

Add support for parenting Layers in a tree. Layers
follow scene-graph style rules, that is to say:
   1. A child is cropped to the final bounds of the parent.
   2. A child inherits the parent's transform (including position)
   3. A child's Z ordering is relative to the parent and bounded between
      the parents siblings.
   4. A childs lifetime is bounded by it's parents lifetime.

Test: New tests in Transaction_test plus manual testing with later branches.
Change-Id: I96f8ad863665b9a70b6f845561344c297b7e6eff
diff --git a/services/surfaceflinger/SurfaceInterceptor.cpp b/services/surfaceflinger/SurfaceInterceptor.cpp
index 60bc7f3..2d6472a 100644
--- a/services/surfaceflinger/SurfaceInterceptor.cpp
+++ b/services/surfaceflinger/SurfaceInterceptor.cpp
@@ -74,9 +74,11 @@
 
 void SurfaceInterceptor::saveExistingSurfacesLocked(const SortedVector<sp<Layer>>& layers) {
     ATRACE_CALL();
-    for (const auto& layer : layers) {
-        addSurfaceCreationLocked(createTraceIncrementLocked(), layer);
-        addInitialSurfaceStateLocked(createTraceIncrementLocked(), layer);
+    for (const auto& l : layers) {
+        l->traverseInZOrder([this](Layer* layer) {
+            addSurfaceCreationLocked(createTraceIncrementLocked(), layer);
+            addInitialSurfaceStateLocked(createTraceIncrementLocked(), layer);
+        });
     }
 }