Converted BFTask properties getters into actual readonly properties.

- Converted `result`, `error, `exception`, `isCancelled/cancelled`, `isCompleted/completed` to readonly properties on BFTask to support KVO/KVC.
- Updated initializers/constructors to use `instancetype`, instead of `id` or `Class *`, when it is defined in the header.
- Added missing imports of Foundation to `BFAppLinkReturnToRefererController.h`, `BFAppLinkReturnToRefererView.h`