Reset the FrameNumber for dropped frame in async mode

In Async, reset the framenumber of frames dropped by the BufferQueue.
This allows the client to dequeue and use this frame before the other
queued frames which may be with the MDP.

CRs-fixed: 448606

Change-Id: Icaabf77b55a7275dc98ba1fdfba89508feb92145

libs/gui/BufferQueue.cpp

@@ -651,6 +651,8 @@ status_t BufferQueue::queueBuffer(int buf,
                 Fifo::iterator front(mQueue.begin());
                 // buffer currently queued is freed
                 mSlots[*front].mBufferState = BufferSlot::FREE;
+                // reset the frame number of the freed buffer
+                mSlots[*front].mFrameNumber = 0;
                 // and we record the new buffer index in the queued list
                 *front = buf;
             }