turn DisplayDevice into a reference-counted object

it's safer this way because this object owns an
EGLSurface which cannot be easily reference-counted.

it also gives us the ability to sub-class it, which
we might want to do soon.

Change-Id: I07358bb052dc5a13b4f2196b2c2b6e6e94c4bb4f
diff --git a/services/surfaceflinger/LayerScreenshot.h b/services/surfaceflinger/LayerScreenshot.h
index 64c0281..c06c23c 100644
--- a/services/surfaceflinger/LayerScreenshot.h
+++ b/services/surfaceflinger/LayerScreenshot.h
@@ -43,7 +43,7 @@
 
     virtual void initStates(uint32_t w, uint32_t h, uint32_t flags);
     virtual uint32_t doTransaction(uint32_t flags);
-    virtual void onDraw(const DisplayDevice& hw, const Region& clip) const;
+    virtual void onDraw(const sp<const DisplayDevice>& hw, const Region& clip) const;
     virtual bool isOpaque() const         { return false; }
     virtual bool isSecure() const         { return false; }
     virtual bool isProtectedByApp() const { return false; }