BufferQueueProducer: fix which slot gets pre-allocated

Commit 78014f32da introduced a bug that
made us pre-allocate buffers into the last available free slots instead
of the first available ones. This in turn caused more re-allocations,
and possibly triggered driver bugs.

Change-Id: Ic4a70e676b4f2bbb054bc873be62ced26e3099a0

libs/gui/BufferQueueProducer.cpp

@@ -890,7 +890,7 @@ void BufferQueueProducer::allocateBuffers(bool async, uint32_t width,
                         continue;
                     }
 
-                    freeSlots.push_front(slot);
+                    freeSlots.push_back(slot);
                 }
             }