Fix a bug in SurfaceTexture::dequeueBuffer.

This change fixes a bug that allowed the most recently queued buffer to
be returned by dequeueBuffer.  At the next updateTexImage call, the
dequeued buffer would be set as the contents of the texture even though
the client could be writing to it.

Change-Id: I53dc14eed13262475627d5551337df57fd78fe00