Bug 1024395 - Lockscreen color overlay not working due to bug 973443

Please refer to https://github.com/mozilla-b2g/gaia/pull/19712/files#diff-39d8fcc7bf1d607a5a5b99794f1d7b69
The anonymous was used to wrap the `onPickSuccess` function so that we could write test for it easily. I would suggest to pass the color to `onPickSuccess` and call to `_setWallpaperColor` in it.