Delegate attribute fix

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.)