CB-2674 Add prompt to Notification API for iOS

- adds native code corresponding to JS added in CB-2679
Remove some unnecessary argument checks in CDVNotification

The JS ensures that all the params are present, and that they are not
nil.
CB-2691: Splashscreen should block user interaction

- this disables user interaction while splashscreen is shown
[CB-2732] Only set camera device when allowed.

The camera device can only be set when the source type is
UIImagePickerControllerSourceTypeCamera (ie. we're actually using the
camera).