Ref count the nodes in VectorDrawable

Also added API check so that for pre-N we ignore invalid VD animation,
in order to avoid breaking old apps.

Bug: 26975469
Bug: 26949340
Change-Id: I498539ad6a05de3d886e7dcdc8a167e78333ab11
diff --git a/libs/hwui/VectorDrawable.cpp b/libs/hwui/VectorDrawable.cpp
index 541c799..2e3856f 100644
--- a/libs/hwui/VectorDrawable.cpp
+++ b/libs/hwui/VectorDrawable.cpp
@@ -324,7 +324,7 @@
     // Save the current clip information, which is local to this group.
     outCanvas->save();
     // Draw the group tree in the same order as the XML file.
-    for (Node* child : mChildren) {
+    for (auto& child : mChildren) {
         child->draw(outCanvas, stackedMatrix, scaleX, scaleY);
     }
     // Restore the previous clip information.
@@ -361,7 +361,7 @@
 }
 
 void Group::addChild(Node* child) {
-    mChildren.push_back(child);
+    mChildren.emplace_back(child);
 }
 
 bool Group::getProperties(float* outProperties, int length) {