bluetooth: Support port availability flag

bluetooth: Support port availability flag

Use the port availability flag to expose whether a certain profile is
connected and whether it's doing actual audio streaming.

The proposed mapping is the following:
- Profile disconnected: port is unavailable
- Profile is connected (but not streaming/playing): availability unknown
- Profile is streaming/playing: port is available

The availability-unknown is specially interesting: it involves that if
the sink/source exists (corresponding card profile set), it is currently
in suspended state.

For example, for SCO cases (HFGW or HSP), this means the SCO is down. A
policy module would typically not change this, unless someone is really
trying to use the sink/source. This situation would be nicely handled by
module-suspend-on-idle, which would automatically connect SCO.

On the other hand, if the user wants to control the status of the SCO,
it will still be possible by resuming the sink or source (suspend=0).
This works out-of-the-box since most UIs would show to the user ports
whose availability is unknown.