Bug 1105639 - Release observers when SettingsHelper is done r=...

SettingsHelper's init() method does add an observer on the setting
value. To avoid leaking, we have to make sure we call removeObserver()
once we are done with the request.
`SettingsHelper` should only be created once. We could use the same instance of `SettingsHelper` to manipulate the value stored under `lockscreen.lock-message`.