SurfaceTexture: fix an error check in dequeueBuffer.

This change fixes the MIN_UNDEQUEUED_BUFFERS error check in
dequeueBuffer.  The check should only be performed if a buffer has been
queued since the last time the buffer count was changed by the client.
The check must be applied conditionally because video decoders require
all the bufferes to be dequeued and registered before beginning the
decode.

Change-Id: I08d96b380544e395c2fcf0f3983a199bfd695b09