playbin: filter out buffering messages when switching uri

playbin: filter out buffering messages when switching uri

When switching URI from about-to-finish, playbin starts decoding the new
URI and the queue2 inside uridecodebin starts emitting buffering messages
immediately. However, the queue(s) inside playsink still have buffers to
play and the pipeline doesn't need to pause for buffering, so we should
not send those buffering messages up to the application, otherwise there
is an audible glitch caused by pausing the pipeline for a very short time.

https://bugzilla.gnome.org/show_bug.cgi?id=727255