Unpin images when skia pipeline object is destroyed

Unpin images when skia pipeline object is destroyed. This fixes
an assert in skia ~SkImage_Raster.

Test: Built and ran Android on a device.
Bug: 66730647
Change-Id: I73603efd4fc2ca584cdb8f6a4774e48a187f6787
diff --git a/libs/hwui/pipeline/skia/SkiaPipeline.h b/libs/hwui/pipeline/skia/SkiaPipeline.h
index 2b0c419..3e6ae30 100644
--- a/libs/hwui/pipeline/skia/SkiaPipeline.h
+++ b/libs/hwui/pipeline/skia/SkiaPipeline.h
@@ -28,7 +28,7 @@
 class SkiaPipeline : public renderthread::IRenderPipeline {
 public:
     SkiaPipeline(renderthread::RenderThread& thread);
-    virtual ~SkiaPipeline() {}
+    virtual ~SkiaPipeline();
 
     TaskManager* getTaskManager() override;