BQ: Prevent operations on disconnected BQs

- Update unit tests to match

Bug 23763412

Change-Id: I77e59bf6b57b328433c3835450455f80a8fa454b
diff --git a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
index 5807eb1..91e94f1 100644
--- a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
+++ b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.h
@@ -108,7 +108,7 @@
     virtual status_t attachBuffer(int* slot, const sp<GraphicBuffer>& buffer);
     virtual status_t queueBuffer(int pslot,
             const QueueBufferInput& input, QueueBufferOutput* output);
-    virtual void cancelBuffer(int pslot, const sp<Fence>& fence);
+    virtual status_t cancelBuffer(int pslot, const sp<Fence>& fence);
     virtual int query(int what, int* value);
     virtual status_t connect(const sp<IProducerListener>& listener,
             int api, bool producerControlledByApp, QueueBufferOutput* output);