Fix set change listener

[Bluetooth] setChangeListener API fix
Now 'AdapterUpdated' JSON message is sent to javascript and
checks listener existence before calling its callbacks.
Do you mean that you want something like below ?
It appears to me weird to 4-space indent again
const char* visible =
    (visibility_mode == BT_ADAPTER_VISIBILITY_MODE_NON_DISCOVERABLE) ?
        "false" : "true";