Add captureLayers function to capture a layer and its children.

The captureLayers function gets a root layer as its argument.
It will capture the content for that layer and its descendants. The
capture will set the root layer's transform back to (0, 0).

Test: Transaction_test ScreenCaptureTest
Change-Id: I84fb66a65cd91434cddc99506b1924cf9f950935
diff --git a/libs/gui/include/gui/SurfaceComposerClient.h b/libs/gui/include/gui/SurfaceComposerClient.h
index 00d5936..d63dafe 100644
--- a/libs/gui/include/gui/SurfaceComposerClient.h
+++ b/libs/gui/include/gui/SurfaceComposerClient.h
@@ -297,6 +297,9 @@
             bool useIdentityTransform,
             uint32_t rotation,
             sp<GraphicBuffer>* outbuffer);
+    static status_t captureLayers(const sp<IBinder>& layerHandle,
+                                  const sp<IGraphicBufferProducer>& producer, uint32_t rotation);
+
 private:
     mutable sp<CpuConsumer> mCpuConsumer;
     mutable sp<IGraphicBufferProducer> mProducer;