Make active Context valid while buildLayer() When RecordingCanvas::drawPoints() is executed through buildLayer(), active Context is null.
It leads to crash

Change-Id: Iefe7fa0e056e82f07cd54fa3bfd74180f89ae726
diff --git a/libs/hwui/renderthread/CanvasContext.cpp b/libs/hwui/renderthread/CanvasContext.cpp
index 504dfaa..cedfaed 100644
--- a/libs/hwui/renderthread/CanvasContext.cpp
+++ b/libs/hwui/renderthread/CanvasContext.cpp
@@ -919,6 +919,7 @@
     // buildLayer() will leave the tree in an unknown state, so we must stop drawing
     stopDrawing();
 
+    ScopedActiveContext activeContext(this);
     TreeInfo info(TreeInfo::MODE_FULL, *this);
     info.damageAccumulator = &mDamageAccumulator;
     info.layerUpdateQueue = &mLayerUpdateQueue;