Add Canary track to Android Studio cask

I took most of this from the regular [Android Studio file](https://github.com/caskroom/homebrew-cask/blob/ea1d3ead46148ecc443daf3dc574c15b19e91452/Casks/android-studio.rb). I'm not Rubyist and I'm not sure about the internals of homebrew and cask, but it seems like there should be a good way to have some type of abstract class/module/mixin for both of these. If so, where's the best place to put that?