bluez5: emit & remove the A2DP source node depending on transport state

Typically a source stops the connection when it has nothing to play
and this causes the transport to become "idle" and our A2DP source
also stops. However, the node is still present and "running" (if linked),
which causes the graph to underrun as it receives no data from this node.

This patch dynamically creates and destroys the a2dp source node depending
on the transport state. So, when the transport is idle, there is no node
in the graph at all.