kiosk: Management UI polish.

- Add an "Add" button on the side of the input field to make it more
  explicit;
- Make "Done" button to add the current value in the input fields
  before dismissing the UI.

BUG=307543

Review URL: https://codereview.chromium.org/26375008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228825 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228825