Add discovery toggle button and connect devices in apps for bluetooth-samples

The handlers here are pretty much identical, so you may be able to simplify this by defining the handler beforehand. Something like:

    var callback = function() {
      if (chrome.runtime.lastError)
        console.log('Failed to ' + (self.discovering_ ? 'stop' : 'start') +
                    chrome.runtime.lastError.message);
    };

    if (self.discovering_)
      chrome.bluetooth.stopDiscovery(callback);
    else
      chrome.bluetooth.startDiscovery(callback);