avdtp: Register AudioSource before agent authorization

Make sure the D-Bus interface AudioSource has been registered and it's
state is 'connecting' by the time the agent authorization is sent.

This patch assumes the remote end supports the A2DP source role, which
is actually not known at this point in time. However, the disadvange of
doing this is significantly less important than not doing so, since the
API has serious limitations otherwise.

With this change, the agent has the guarantee that IVI audio interfaces
will already be in 'connecting' state by the time the agent
authorization is sent.