Bug 945631 - Listen to event discoverystatechanged

After Bluetooth backend has changed to Bluedroid in Firefox OS 1.3, the discovery session will be terminated in about 13 secs (and I haven't found where I could modify this number without touching code). Under certain circumstances, Gaia needs to handle event discoverystatechanged which would be sent once the discovering state changes, otherwise UI won't react as expected.