Refactor KVO in categories

Smashing `observeValueForKeyPath:ofObject:change:context:` in categories is a very bad idea. :scream_cat:

Here’s my progress so far to eliminate this behavior.

- [x] Remove KVO in UIActivityIndicatorView+AFNetworking  
I used notifications instead of KVO.
- [ ] Properly handle this case

```objc
[activityIndicatorView setAnimatingWithStateOfTask:taskA];
[activityIndicatorView setAnimatingWithStateOfTask:taskB];
```
- [ ] Remove KVO in UIProgressView+AFNetworking