libgui: Plumb detachBuffer through ConsumerBase

Exposes IGraphicBufferConsumer::detachBuffer as a ConsumerBase
method. attachBuffer is not currently exposed, because all current
clients will be recycling buffers through the allocator.

Bug: 19628705
Change-Id: I3e519767fa43d5d880c1d5695e31b60f6ad588af
diff --git a/include/gui/ConsumerBase.h b/include/gui/ConsumerBase.h
index ef0feab..d56fa89 100644
--- a/include/gui/ConsumerBase.h
+++ b/include/gui/ConsumerBase.h
@@ -76,6 +76,9 @@
     // when a new frame becomes available.
     void setFrameAvailableListener(const wp<FrameAvailableListener>& listener);
 
+    // See IGraphicBufferConsumer::detachBuffer
+    status_t detachBuffer(int slot);
+
 private:
     ConsumerBase(const ConsumerBase&);
     void operator=(const ConsumerBase&);