Fix a SurfaceFlinger bug.

The bug caused SurfaceFlinger to constantly trigger reallocation of buffers
that requested a fixed size.

Change-Id: Ic993a60e6474e36a344c8b48d541a0d08b8c4faf

services/surfaceflinger/Layer.cpp

@@ -380,6 +380,7 @@ sp<GraphicBuffer> Layer::requestBuffer(int index,
             mReqWidth  = reqWidth;
             mReqHeight = reqHeight;
             mReqFormat = reqFormat;
+            mFixedSize = reqWidth && reqHeight;
 
             lcblk->reallocateAllExcept(index);
         }