Emit state-changed signal when an uriendpoint changes its state

Recorder endpoint is not able to change its state directly. For example,
when recorder transites to STOP state it needs to flush out its
biffers before transiting. When the change has taken place, the proper
signal is emited so as to announce the change to application developers.

Change-Id: I82fe309fc7d220e31f68cd20193257b03e338aa0