workaround a deadlock when taking screenshots into a surface

When disconnecting from BufferQueue, we now drain the queue
except the head (which means in the screenshot case we won't
have to block, but we might not have a buffer to show, this
will appear as an error in the log).

Bug: 8362363
Change-Id: If80989aac3c917beea2ebddf3cbb502849d394da