Pass the IGraphicBufferAlloc to SurfaceTextureClient.

This change passes a reference to the IGraphicBufferAlloc binder object
to SurfaceTextureClient objects.  When STC objects are created they
query their associated ISurfaceTexture object for the
IGraphicBufferAlloc that the SurfaceTexture uses to allocate buffers.
Having the SurfaceTextureClient hold this reference prevents the
GraphicBufferAlloc in SurfaceFlinger from freeing the allocated buffers
before the SurfaceTextureClient is done with them.

Change-Id: Ib8e30e8b37fdd60438cbb4cb7e9174d0ba6d661c
related-bug: 3362519
diff --git a/include/gui/SurfaceTextureClient.h b/include/gui/SurfaceTextureClient.h
index dd1d490..ff2251d 100644
--- a/include/gui/SurfaceTextureClient.h
+++ b/include/gui/SurfaceTextureClient.h
@@ -83,6 +83,10 @@
     // interactions with the server using this interface.
     sp<ISurfaceTexture> mSurfaceTexture;
 
+    // mAllocator is the binder object that is referenced to prevent the
+    // dequeued buffers from being freed prematurely.
+    sp<IBinder> mAllocator;
+
     // mSlots stores the buffers that have been allocated for each buffer slot.
     // It is initialized to null pointers, and gets filled in with the result of
     // ISurfaceTexture::requestBuffer when the client dequeues a buffer from a