BufferQueueProducer: fix which slot gets pre-allocated

Commit 78014f32da6d0ebf52fb34ebb7663863000520a0 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
diff --git a/libs/gui/BufferQueueProducer.cpp b/libs/gui/BufferQueueProducer.cpp
index cdc810d..a53775f 100644
--- a/libs/gui/BufferQueueProducer.cpp
+++ b/libs/gui/BufferQueueProducer.cpp
@@ -890,7 +890,7 @@
                         continue;
                     }
 
-                    freeSlots.push_front(slot);
+                    freeSlots.push_back(slot);
                 }
             }