BQ: Prevent operations on disconnected BQs

- Update unit tests to match

Bug 23763412

Change-Id: I77e59bf6b57b328433c3835450455f80a8fa454b
diff --git a/services/surfaceflinger/MonitoredProducer.cpp b/services/surfaceflinger/MonitoredProducer.cpp
index 85182d9..f16ad59 100644
--- a/services/surfaceflinger/MonitoredProducer.cpp
+++ b/services/surfaceflinger/MonitoredProducer.cpp
@@ -89,8 +89,8 @@
     return mProducer->queueBuffer(slot, input, output);
 }
 
-void MonitoredProducer::cancelBuffer(int slot, const sp<Fence>& fence) {
-    mProducer->cancelBuffer(slot, fence);
+status_t MonitoredProducer::cancelBuffer(int slot, const sp<Fence>& fence) {
+    return mProducer->cancelBuffer(slot, fence);
 }
 
 int MonitoredProducer::query(int what, int* value) {