Bug 1070823 - [Bluetooth][Settings] bluetooth panel support BT v2 API

nit: Let's chain the functions instead of making them nested as we are using promises:
```
this._defaultAdapter().then(function(adapter) {
  return adapter.setDiscoverable(enabled);
}).then(() => {
  this.discoverable = enabled;
}, function(reason) {
  console.log('--> set discoverable failed: reason = ' + reason);
});
```