Fix sideband stream issues

- Notify a listener when sideband stream is set
- Mark a layer as visible when sideband stream is set, even though
no buffer is queued.

Change-Id: I9652bf530f2b5ce331533ec1bb3b10a815ca191c
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 7f2ee2a..f272fd2 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -959,7 +959,7 @@
 bool Layer::isVisible() const {
     const Layer::State& s(mDrawingState);
     return !(s.flags & layer_state_t::eLayerHidden) && s.alpha
-            && (mActiveBuffer != NULL);
+            && (mActiveBuffer != NULL || mSidebandStream != NULL);
 }
 
 Region Layer::latchBuffer(bool& recomputeVisibleRegions)