Update delegate property attribute from assign to weak

ASMediaFocusManager has been crashing for us on line 321 of ASMediaFocusManager.m with EXC_BAD_ACCESS KERN_INVALID_ADDRESS. The property `delegate` currently has the `assign` attribute which means that if it's deallocated, we are left with a dangling pointer under ARC. This pull request changes the property to `weak`. (This reasoning based on the "Property Attributes" of [Apple's ARC transition docs](https://developer.apple.com/library/ios/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html).)