Loads of visual glitch fixes. Other small improvements

Fixed the following issues:
The snapshot used to animate the resume of the Facebook app used to be just black with chat heads on top.
The app would go all black with chat heads on top when you're in the app and do something like open the app switcher or notification centre.
Swiping up to close Facebook with Zephyr would just leave you swiping a transparent view with a shadow.

Added the following improvements:
Chat heads now animate in on unlock.
Chat heads animate out when the app switcher opens (to reduce confusion since you couldn't interact with them anyway).
Chat heads can be dismissed with the home button when outside of the Facebook app. 
Opening a chat head while home screen icons are in wiggle mode disables wiggle mode.