Expose availability from the backend to C++ and QML.

The availabilityError property was static based on the service, but it
can change at run time, so add the plumbing to allow the backend to
report it itself.

Also make sure that both QML and C++ expose the availability.

The radio tuner and data controls previously had properties (but no
signals) for availability - these have been removed.

Change-Id: I9240cf93e2a51b14cd38642f9312ae3c75f05361
Reviewed-by: Ling Hu <ling.hu@nokia.com>

examples/declarative-radio/view.qml

@@ -72,7 +72,7 @@ Rectangle {
                 height: 200
 
                 verticalAlignment: Text.AlignVCenter
-                text: (radio.isAvailable() ? "No Signal " : "No Radio Found")
+                text: (radio.availability == Radio.Available ? "No Signal " : "No Radio Found")
             }
         }