Bug 1038984 - [L10n][Gallery] Clean up mozL10n.get uses in Gallery app

Video app currently uses 'empty-title/text'. I suggest changing that to 'emptyvideo-title/text' to be consistent with gallery and then add the following 'empty' case:
        case 'emptyvideo':
          title = 'emptyvideo-title';
          text = 'emptyvideo-text';
          $('overlay-menu').classList.remove('hidden');
          if (pendingPick) {
            $('overlay-cancel-button').classList.remove('hidden');
          } else {
            $('overlay-camera-button').classList.remove('hidden');
          }
          break;

Also, change this line: https://github.com/mozilla-b2g/gaia/blob/master/apps/video/js/video.js#L674

to 
showOverlay('emptyvideo');